The standard multi-day arc is preview night, full-price public days, half-price day, then bag day. Some sales skip half-price and go straight to bags; larger sales may add a members-only cleanup hour at the very end.
Half-price day is the sweet spot for many shoppers. Selection is still broad enough to find complete series and clean copies, but the price per book has dropped enough that marginal picks become worth taking.
Watch for category exceptions. Better books, DVDs, puzzles, and vinyl are often excluded from markdowns, and some groups reprice rather than discount, so the sticker you saw yesterday may not be the sticker today.