When is the next guaranteed gacha?

They're all slightly different. The upcoming New Year's one is a set of banners for each class. You can only roll on one banner and then they all disappear. When you roll on a GSSR banner you're for sure getting 1 5* servant. The other servants featured in them are on rate-up though so you have the normal odds of getting more than just the 1 SSR. The minimum amount of money you need to spend to roll is around 30 dollars as you need at least 30 paid quartz.