A ridiculous claim.
There are basically fourteen great and ancient feasts of the Latin Church, categorized under the old system of feast days as "Doubles of the First Class" or the holiest days of the year.
There are more than fourteen, but a number of them are quite recent, such as the feast of the Kingship of Jesus Christ which is less than 100 years old.
Of the twelve ancient feasts, five are moveables: the Resurrection, the Ascension, Pentecost, Corpus Christi, and the Blood of Christ.
All of them are timed in conjunction with the Jewish Passover.
Then there are the feasts of the Dormition of Mary, of St. John the Baptist, of St. Joseph, of Peter And Paul, of All Saints and of Michael the Archangel.
Of these six, St. Michael falls a week after the autumnal equinox and All Saints falls a week after the Celtic festival of Samhain.
The remaining feasts are Christmas, the Circumcision and Epiphany.
Epiphany and Circumcision are timed specifically to Christmas and Christimas falls several days after the winter solstice or Saturnalia.
So, out of fourteen feasts, three fall on days which are roughly the same time as some pagan festival or other.
In the case of All Saints, the celebrations associated with Samhain are completely different from the cultural practices associated with All Saints. Moreover, the feast dates from the specific dedication of a church of All Saints in Rome in the 600s - it had no relation with Celtic festivals.
The feast of St. Michael a week after the autumnal equinox coincides with no European pagan holiday of any kind. There is a Persian holiday on the autumnal equinox, but it's difficult to see any influence.
The supposed relationship of Christmas and Saturnalia is one I've already gone over.
Of all the major Christian feasts, not one can be credibly linked to pagan festivals.
The fact that you have to scrape up extremely minor holidays like St. Valentine's Day, which was never a major feast, is instructive.
The Christian calendar is based on two sources: (1) Passover and (2) historical events in the Christian Church - like the beheading of John the Baptist, the crucifixions of Peter and Paul, etc.
Honestly, all in all, it doesn’t mean a thing to me if people danced in the woods or sacrificed children on the same days as Catholics celebrate Holy Days.
This is where intent comes in. I have statues in my church but I don’t pray to statues. Personally, I don’t care if others think I do. My intent is not to do that.
And if one of my Feast Days falls on a made up Wiccan holiday or a pagan holiday, who cares? It’s not about them.
Christ came to replace all the silly worship of pond scum and such. So He did.
The events commemorated by the latter feasts are not observed on the actual anniversaries (since these are not known) but ultimately on a didactic use made of the yearly cycle. Eg, no one believes that J*sus was actually born on 12/25. This time of year was chosen, if not as an inculturation of earlier pagan holidays, because of the didactic value of celebrating light coming into the world on the darkest night of the year. The Jewish holidays, on the other hand, occur on the actual anniversaries of the events commemorated.
As to the computation of the Jewish calendar, it is true that the current method of using a fixed calendar dates back to Hillel II and that prior to that time the new moon was determined by two witnesses, in the following manner: On the thirtieth day of each month if two witnesses testified to the Beit Din that they had seen the new moon, then that day was declared the first day of the new month. If no such witnesses appeared the following day automatically became the first day of the new month. (Note that the new moon sacrifices were offered on the morning of the thirtieth since there was always the possibility that it would be the new moon. When it was not they were offered again the next morning.)
The actual molad (instant of the "birth" of the moon) was actually known beforehand based on mathematical principals. However, this was not used; the witnesses were used instead. So the molad itself (the method for finding which is encoded in the opening passage of the Torah) is not the product of recent computations. With some modifications, this it what is used today (modifications were required to prevent Yom Kippur from falling on the day before or after Shabbat or Hosha`-na' Rabba' from falling on Shabbat; this means the new year cannot begin on the first, fourth, or sixth day of the week).
But more important than all this is that the lunisolar Jewish calendar dates literally back to the creation of the world. The Greco-Roman church merely adapted the pagan Roman solar calendar (other churches, such as the Coptic, Ethiopian, and Syrian churches, use different calendars).