ZOS usually has three crown sales a year, and they're always around the same time: one near the anniversary in April, one for Quakecon in August, and one for the holidays.

Obviously they don't give dates until it happens because they want people to buy crowns, not wait and then stockpile them when they're on sale. These are also typical but not guaranteed, especially not since Endeavors will start to cut into the Crown store profits.

...though if I were ZOS's marketing team, I would spring for more promotions like the Hailcinder elk they did, where you get a free item for buying a pack. That way they get the crowns at full price, nobody can use stashed crowns to get the item, and then people have crowns that they feel obligated to spend in the store. If they found some things that were hotly requested on the Crown forum (like hairstyles) and did that, they would make a killing.