It's usually the 15th of the month but sometimes different, like this month it was the 17th so it fell on a Monday i believe.
Best thing to do is check their news page. Scroll a bit and you can see the post relating to October's sale, and they have a link to send you reminders.
The time is always 9am PST so you can use time and date's los angeles link to check what time it'll be for you.