I would assume it is when they sell their team horses in preparation for that particular week's Grand Prix. For example certain top teams release their teared team horses for Monday's Grand Prix whether that be for a classical or western GP event.

From my experience, breeder's week is when teams tear out extra max hits for their teammates to have a skiller for, instead of solely the hits they need for orders and direct sales.

Breeders week is when the team members are allowed to keep their best teared foals for personal use to sell (without sharing passes with team mates) or participate in the Grand Prix. The other 3 weeks of the month the best teared foals are strictly to fill orders and extras go in direct sales. This keeps things fair for the buying public knowing that 3 out of the 4 weeks of the month they aren't wasting their passes only to be beat by the team.

Crystal Coven is correct ^^ Teams typically tear for orders for Prix, but Breeder's Week is the one week of the month where team members receive the best preps and enter Prix/comp for rosettes.

Thanks everyone <3