Hi Hermione,

Thank you for your efforts - I looked at the same Lockyer publications as well as several journal articles available on archive.org and couldn’t find anything referencing the Sed festival at all.

I have found the following which each indicate the possibility that the sed festival timing is based on the orbital period of Saturn:
Sellers, J., The Death of Gods in Ancient Egypt, Penguin Books, 1992, p218
Kelley, D.H. And Milone, E., Exploring Ancient Skies: An Encyclopedic Survey of Archaeoastronomy, Springer 2005, p261
Talbot, D., The Saturn Myth

Regards,
Brendan