If you were to have multiple triggers at the same moment (ie at the beginning of your upkeep or the beginning of your end step), how is it decided what triggers first?
I've always thought they'd happen in order of what entered the battlefield most recently backwards to what entered first.
When multiple triggers have to be put on the stack at the same time, all waiting triggers are put there in APNAP order (Active Player Non Active Player). Meaning, the active player (the player whose turn it is), puts all his triggers on the stack in the order of his/her choosing (selecting modes and targets as nessessary), then the nonactive players in turn order do the same. So the active player's triggers always resolve last.
Note, that triggered abilities don't have to trigger at the same time for this to occur. There are many scenarios, where triggers go of at diffeent times, but are still put on the stack at the same time. This is because triggers wait to be put on the stack until a player would receive priority, so if for example a resolving spell has multiple effects in sequence, and two of those effects each trigger an ability, those two triggers make it to the stack at the same time, even though they triggered at different times.
If a player controls multiple abilities that trigger at the same time, that players chooses the order that they are put on the stack.
If multiple abilities trigger at the same time for different players, then they are put on the stack in Active Player - Non-Active Player (APNP) order. So for example, if it is your turn, and you and your opponent both have something that triggers on your upkeep, you will put your triggers on the stack first, then your opponent will put theirs on the stack above yours.
Two or more abilities can trigger at the same time, but after they do, they go on the stack at the same time (C.R. 603.3b, 603.3). Specifically, "each player, in [Active Player, Nonactive Player] order, puts triggered abilities he or she controls on the stack in any order he or she chooses" (C.R. 603.3b; that rule cites C.R. 101.4).
I've always thought they'd happen in order of what entered the battlefield most recently backwards to what entered first.
Note, that triggered abilities don't have to trigger at the same time for this to occur. There are many scenarios, where triggers go of at diffeent times, but are still put on the stack at the same time. This is because triggers wait to be put on the stack until a player would receive priority, so if for example a resolving spell has multiple effects in sequence, and two of those effects each trigger an ability, those two triggers make it to the stack at the same time, even though they triggered at different times.
Former Rules Advisor
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If multiple abilities trigger at the same time for different players, then they are put on the stack in Active Player - Non-Active Player (APNP) order. So for example, if it is your turn, and you and your opponent both have something that triggers on your upkeep, you will put your triggers on the stack first, then your opponent will put theirs on the stack above yours.
Hope that helps.
EDIT (Jun. 15, 2022): Edited slightly.
Glad that's cleared up