Easter of course, preceded by another fast, Lent, which lasted from Ash Wednesday until Easter. shrove Tuesday, the day before lent started, was traditonally a day for a feast before the fast started, pancakes were traditionally eaten on that day, and still are, even though fasting in Lent is a custom that has mostly lapsed in england.
May Day is still celebrated somewhat here, though more so in other countries. Bringing home may blossom to decorate houses used to be a big thing here, and it was a time for merrymaking and (traditonally) a time of sexual license.
Whitsun and corpus Christi (in June) used to be big festivals, but again have fallen into disuse in england, though they are still big in Catholic countries, Spain has a big Corpus Christi festival. Midsummer is still a big celebration in Nordic countries, i think it's avery big festival in Sweden etc.
Lammas, at the beginning of August, is another festival that has fallen into disuse in england, but I think is still celebrated in a big way in Ireland.
And we still celebrate harvest festival here in england, at the end of September, although we are no longer a very rural nation, and harvest doesn't really mean a great deal to the English as a whole any more.
I think most festival mark the different seasons of the year, or remind us of particular religious events, as with Christmas and Easter. And they give people a reason to have fun.