Im having a little misunderstanding considering this card. I understand the part about kneeling 5 influence or 2 nobles to play this card during the plot phase. But, the part about (If it is summer, you may play this card during the challenge phase). Are there plot or event cards that specify "seasons"? Or does it mean literally summer?

It means there needs to be a Black Raven in play. There is also a White Raven that makes it Winter. There are other potential ways to make it summer or winter, but these are the 'main' way to do it.

Be aware that both the chapter packs these cards come in are out of print and can be very difficult to find, at least for me anyway.

The crowns also make it summer but martell doesn't have one.