What does 'when you play' mean on cards like First Mate, and how often can it trigger per turn?

Ruling: 'Playing' a card means putting it on the board into play from your hand or other zone. Moving cards that are already on the board (such as moving to a battlefield or base) does not count as playing them.

Sequence:

• A card is played when it enters the board face-up
• Hidden cards are not considered played until they are flipped face-up and used
• The 'when you play' ability triggers at that moment

Nuances:

• A card can only trigger its 'when you play' ability again if it leaves the board and is re-played (e.g., returned to hand then played again, banished and re-entered, or brought back from trash)
• There is no inherent limit to how many times a card can be played in a turn, but it requires removing it from the board and playing it again
• In most situations, cards are only played once per game