The banner for the last Monday sale I noticed on 7/13 went up at midday game-time after I reloaded the game, today (Monday 7/27)'s Flash Sale banner showed up after I reloaded my game at midday as well. The only reason I reload at that time is for the Cabbage switch-over. 😂 Not sure if it's a consistent/planned schedule, but i'd say reload Mondays after "midday game time" but before... (or at least with sufficient time to research and purchase the item if you're interested before) 7pm (since they're 7 hour flash sales?) to be sure! 😊