"Civilized men are more discourteous than savages because they know they can be impolite without having their skulls split, as a general thing.”
― Robert E. Howard
Search online and you can find much better prices than buying from the ESO store (which is grossly overpriced).
I paid around $12 for the base game when the website had it for like $30, got a copy of the gold edition for about the same, picked up Morrowind last fall for around $15, and just bought Summerset for another $15.
Black Friday is your best bet. There's no guarantee - they never tell us far in advance when it's on sale (probably because then no one would buy it at full price) but that's the most likely time.
Keep checking the news page and Facebook/Twitter over the next few weeks and don't assume the sale will be on the actual day.