I had Maester of the Sun in play and it was summer.

If I lose a military challenge,

may I kneel Maester of the Sun and discard a character card to sve him, and stand him due to the Vengeful effect?

spring10345 said:

Since the "save" opportunity for claim comes before passive effects (like Vengeful) are resolved, you can indeed use the Maester's ability to save a character from a lost military challenge, then stand him (almost) immediately with Vengeful for losing that same challenge.