Whenever this creature becomes untapped, you may have target player discard a card.
If Hollowsage becomes untapped during your untap step, the ability will trigger. However, since no player gets priority during the untap step, it waits to be put on the stack until your upkeep starts. At that time, all your “beginning of upkeep” triggers will also trigger. You can put them and Hollowsage’s trigger on the stack in any order.
