Pinkie invites Rarity over every Thursday to help her with something. To "Eat Cheese" as Rarity wants to put it.

WARNING: This is a silly parody of (Likely a much better story) Monochromatic's "One Wednesday"
https://www.fimfiction.net/story/564505/one-wednesday

Honestly, you're probably better off reading that. However it gave me this idea due to some play on words shenanigans, and well here we are. I'm sorry I couldn't resist. I don't blame you if you hate me. XD