Every day, there are designated dungeon bonus events including 1.5x drop rate, 2x coin rate, 1/2 stamina cost, etc. The times listed in this article are all PST/PDT.
All Technical Dungeons feature 1.5x drop rate on Mondays (starting at midnight PST). This is an excellent opportunity to farm for Pengdras.
Special dungeons will get bonuses on weekends, limited to "Biweekly Dungeons". A Biweekly Dungeon will get 2x drop rate on the Saturday of its first week and Sunday of its second week, from 12PM to midnight PST.
Other special dungeon events will be announced by GungHo separately.