Who first decided the date of festivals in Calendar?

I can only speak for the western world, but I think it would be sensible to assume that events and dates of these events decided when festivals were held, that is, to celebrate or remember key happenings such as the date someone famous died or was born or won some battle. The exception is religious festivals, and it seems that these tend to coincide with weather changes of the year.

In past times, the coming of Spring after the misery of winter was a time to celebrate new growth. This was the time of year when sheep lambed, as well as buds appearing on the tree. It was appropriate for the rising of Christ to be aligned with this, and Easter became an established festival. This is an example, Christmas, harvest being others.