"Much of our calendar is to do with the working out where Easter falls as it is a moveable feast unlike say Christmas Day. although that also is based on lunar events and counting days (not including Sundays I believe), to correctly celebrate Lent, Good Friday, Palm Sunday, etc."

Yep. Much of calculus was driven by this need, yet I could never see entirely why. The date for Easter could easily be worked out using a stone row aligned east-west. Easter Sunday is always the first Sunday after the first full moon after the Vernal Equinox. So, once you know when the Equinox is (with the help of your stone row) you can work out when Easter is.

I would imagine that the original pre-Ctristian feast was on the day of the full-moon or the day after.