First and foremost, what people need to comprehend, and which most scholars and academics have gotten into the foolish habit of, is referring to everything Pre-Christian, including Greek and Roman cultures as Pagan and Heathen. No one called themselves that. It was after the various forms of Christianity that came about later having expanded from a bunch of fragmented mystery cults and became an officiated version as the State Religion of the Roman Empire that the whole thing got mixed up, reversed and imposed.
Its then from these very same so called "Christian Sources" that so called classical and New Pagan Heathens build all their modern fictional nonsense from. So, in reality, Judaism, Christianity, Islam or whatever didn't "steal" anything. Christianity, in fact and primarily, invented Paganism, heathenry and Satanism as well as the various mystical and occult nonsense that is at the core of the filled in with gnostic rhetoric and nonsense. Most of that, as such, came about mostly in the late 1800s and more so by the 1950s where everything began to mutate further into deeper nonsense thereafter.
That being said, all the various so-called Christian Holidays being claimed to be Pagan stems from the nonsense infighting between Protestants and Catholics. It's idiotic yet true. Having stated this much, let's start talking about some Catholic/Christian based holidays or observances. The four main ones here that cannot be traced by to Pre-Christian sources are Valentine's Day, Easter, Halloween, or Christmas.
Valentines Day:
The earliest reference to this holiday was in the 13th century CE in Britain and France. The association with Cupid was an addition based on a claim by John Lydgate in the 14th century fir which there are no other sources. Its existence is centuries later than other suggested or claimed sources. In fact, the observance is in actuality the festival of this Saint associated with a concept of Courtly Love. Many try and link it to the observance of Lupercalia traditionally observed on February 15th because the observance of St. Valentine is February 14. This claim of connection was invented in the 18th century CE. Other than the days being so close, there is nothing in practice, symbolism or representation that remotely connects these two distinct celebrations. Most of the customs we see included in this particular season are organic evolutions in both a religious and secular sense, and the fact so many are so willing to impose a wide range of deceptions on such matters as these is also more than frustrating.
Easter:
Easter simply means Eastern. It comes from Greek roots as Austeros, the direction of the rising sun. It is not the name of an invented character called Ostara as an Old Saxon female deity. This, along with many more fictions was invented by someone known now as St. Bede in the 700s and picked up by much later sources pushing and inventing their own nonsense spins on this after silly speculative claims made by Jacob Grimm which others imply took up without researching the matter in their own willful ignorance.
We do know that the concept of using decorated eggs that were hard boiled goes back to 1270 where it is mentioned that King Edward the 1st had given over 400 such gold leaf covered eggs to members of his household as part of this observation which evolved over time to coloring eggs. The custom was based on those to Lent as a time of fasting, so such eggs were boiled so thy would keep during this time and could be eaten safely on Easter Sunday.
There is no mention of a Hare or Rabbit mentioned in association with this time of the year toll after the 1600s which coincides to when Buddhist symbols such as a triangulated image of three Hares appeared, along with the Rabbit bearing a basket. The image of a triad of Hares represents the gentler qualities of the Hare which are selflessness (compassion), respect (piety), and giving (charity). This is also when the Jade Rabbit with a basket is mentioned and associated with the dark crater parts of the moon.
Halloween
This one should be obvious it isn't that ancient either. The name Halloween comes from All Hallows Eve literally meaning all healed and all whole and all holy eve, as in the day before the Catholic Observance of All Saints. The association with so called Samhain is largely fiction because the first mention of it is from the 9th century CE manuscript 'Martyrology of Óengus' and places this so-called Samhain on November 1st. All it says of the time is "This is the day of Stormy Samhain (Sowen)." It's another spelling of Samain which simply means Summer, not Summer's End or it would be spelled Samain Crích in Old Irish, and Simmer Críoch in Old Scottish. This of course does have connections with the Wild Bore variously called Sowan or Sow-ain, Sowin, Soween, Sawen, Sowun and Souin, called a Swine from if male or Sow if female. This specific connection is overlooked frequently whether intentionally or not. Wild Boar and Swine associated with the Sun is actually a known historical fact at least. It is known this was a time to slaughter extra animals, especially swine for meat and of which a portion was "given to the dead" as there were many such customs not unique to this time of year.
Even in this 9th century document there is no mention of dressing up in costumes, making offering to or calling upon the dead, or any of the things commonly claimed and associated with Halloween. The 10th century CE document known as the Sanas Cormaic never mentions Samhain at all, much less numbered it among the "Celtic" observances. It is not till the 1600s that any "ritual or religious" association with Samhain is mentioned, namely by Geoffrey Keating (Irish: Seathrun Ceitinn 1569-1644 CE). He was more or less a Pseudo-historian making things up and basing such things on claims from much later "folk traditions" which were themselves made up by others still. Though he is called a historian, his histories are more fantasy than anything else.
It was Sir James Frazer (1569-1644 CE), a Scottish folklorist who invented much of the claims that "Halloween" was called Samhain, that it means Summer's End, and that it was a time so called "pagans" made sacrifices to the ancestorial dead. Before him was John Rhys (1840-1915) who went as far as rewriting the Sanas Cormaic to support his claims. Others in America proposed and proclaimed that "Samhain" was named after a fictional deity called Saman (which is where the word Shaman comes from and means same one(s) used as a singular and plural) and a known fiction. What we have here is "retro-projection of presentism" onto the past that never existed previously.
Christmas
Literally Anointed Mass from Christ-mas. It has often been claimed to be associated with Saturnalia, the celebration of Saturnalia, which honored the Roman god Saturn was never observed on December 25. It started on December 17-23rd of December. That is at least the claims from the 5tyh century CE by Macrobius. Note that the old Calendar this was based on was discontinued about 45 BCE wherein the Julian calendar was imposed by the emperor it is named after. Its observances were nothing like Yule and didn't include the usage of evergreens or any of the other made-up nonsense. Sol Invictus was observed on December 25, though much of the specifics are completely different. IN fact, it isn't until 354 CE that any Pre-Christian deity is associated with sharing such a birthday in actual historic records. Part of the fallacies were also largely introduced as battles of claims between Arians and Catholics and then later Protestants and Catholics with Protestants being the same source of the claims in the 1700s for the Young and Flat Earth garbage.
This whole debate all centers around Christians trying to sort out the birth of Jesus, is a straightforward deduction from Scripture, analyzed in conjunction with historical knowledge about those who worked in the temple. In other words, it was derived from considerations having nothing whatsoever to do with Roman festivals or any other December-related cultural or non-Christian religious activities.
To get to the point, the general view has been handed down that St. Luke also recorded how the Archangel Gabriel told Mary that Elizabeth was six months pregnant with John (Lk 1:36), which means the Annunciation occurred March 25, as we celebrate. Nine months from March 25, or six months from June 24, renders the birth of Christ on Dec. 25, our Christmas.
Christmas trees, Yule logs, Holly and ivy don't come from Saturnalia which was originally about temporary social upheavals and reversals. As far as the act of giving gifts, this is a very basic and normal human tendency as a sign of care and appreciation and does not have a particularly "religious" connotation. If that is the basis of various complaints, such complaints are being made by idiots. It is that simple.
The word Yule itself, as it is also called, comes from the variants of ġiell, giul, ġeōl, and jól as well as gelos. It’s of Greek to Latin origin meaning ultimately ‘Yell’ in a sense of cheering, and shouting/cheering. Much of the claims of various other associations come after the 1200s such as all the customary decorations of this season from wreathes (actually a Christian symbol and stand in for a crown of thorns using holly and leaves) and garlands being a later addition, trees representing the Tree of Life and lights representing stars originally being candles was introduced around the 1700s, and mistletoe which was an import from Icelandic sources from the 1200s associated with questionable representations of "pre-Christian" so called "mortal deities" renamed later in the 1400s as a set of collections called the Eddas, are all part of the decor that is common this time of the year festivities.
Reindeer and Caribou to the traditional outfits of the Sami people also inspired much of the later remade imagery in a more mocking manner as outfits of "elves" later associated with a remade version of Saint Nick turned into Santa (which has nothing to do with the word Satan). So, one way or another, the reality is all these so-called pagan-heathen-satanic in origin and one can state factually when investigating actual history that claims Christianity "stole everything" is nonsense because of what was invented as expressions thereof, and the other factor that many ancient people simply brought with them their basic symbolism and adapted such to their newfound religion/ways.
In Conclusion'
Celebrate what you will or won't, don't demand or force it on everyone else what you choose to like or dislike because the world does not revolve around you, and don't take out your own idiotic choices on your kids because of the brain rot of endless misinformation perpetuated on all sides. In other words, get over yourself, get your facts straight and do actual research, but don't just continue to regurgitate whatever because your research skills or efforts are deficient. Shut up, learn and move on.