To think of the customs in relation to the current calendar is not always helpful. The pre-Roman calendar if it existed is unknown but was undoubtedly based on Astronomical events such as the Solstices (the two points when the day is longest and shortest) and Equinoxes (day and night of equal length), the cycle of the moon and the seasons.
Superimpose on this natural calendar the Roman influence of Julian Caesar (Julian Calendar from 45 BC onwards of 12 months of 30 or 31 days except February, beginning January 1st).
In around 150 AD, the Christian Church decided to celebrate Christ's birth at the old feast of Saturnalia around the Winter Solstice. This meant a date of conception of 25th March and the Year started on this date.
Easter was already related to the Vernal (Spring) Equinoxe and due to the slight inaccuracy in the Julian Year, was drifting towards Summer. Pope Gregory declared a new more accurate year and to correct past errors decreed that Thursday 4th October 1582 would be followed immediately by Friday 15th October 1582.
Gregory also moved the New Year back to the 1st January. Most Protestant countries ignored the change. England continued with the old calendar and only in 1751 did the change of New Year to 1st January and the adjustment of 11 days (2nd September followed by 14th September) happen. This gave rise to the famous 'Give us back our 11 days' - only 200 years later than originally envisaged.
France typically adopted a French Republican Calendar between 1793 and 1805 of 12 months with 30 days each (3 weeks of 10 days). It didn't work but did give us Thermidor (the hottest month) remembered now by Lobsters. Interestingly, the month names reflected the season characteristic much as the Celtic one had.
The church slowly blended traditional Pagan celebrations with Church Festivals (Such as Halloween / All Souls).
The activities on Calendar Custom days often involve Display Dancing (such as Morris, Molly, Garland) and visiting (such as Souling and First Footing). The ones closest to the Celtic tradition involve Fire (such as Halloween / Bonfire Night and Yule). In general, the surviving customs are fragmented and what looks unrelated at first glance may represent different remnants of the same thing although proof is hard to obtain and much remains conjecture.