Glad you brought this up, Seme.

I agree with being able to vary festivals from year to year as it would develop into a strain having to recreate the same one endlessly. There are lots of ancient festivals though many of them were local rather than encompassing the whole country.

We should not forget the Passion Plays of Osiris at Abydos in this respect. They were celebrated annually and people travelled from all over then country to take part. They go back to at least the Middle Kingdom and considering the importance of Abydos already in the Predynastic Period, it´s likely they were celebrated in some form or other way back in time. And they stayed on into the Late period too. Weighty candidate ;)

Moveable festivals - good idea and makes me think immediately of the Beautiful Reunion, when Hathor travelled from Dendera to Edfu to visit Horus.

There´s an ancient calendar with lots of festivals here.