Maintenance for the week of November 4:
• [COMPLETE] ESO Store and Account System for maintenance – November 6, 9:00AM EST (14:00 UTC) - 6:00PM EST (23:00 UTC)
They never announce it ahead of time (for obvious reasons), but a tentative guess based on past examples would be April or August (and then December/January).
The sales have always been in April (around the anniversary), Auguts (Quakecon), and December (holiday sales). We can't bank on it, but it's likely we'll see that again.