So for like a 13-wk (3 month) bill this Thursday the next batch of bills will become available for auction that closes the following Monday.

Schedule is located here:

https://treasurydirect.gov/auctions/upcoming/

They don’t show up on Fidelity until the auction opens. Note you don’t have to bid, just enter how much you want in “face value”, then after the auction close you find out the exact cost (and thus the actual yield)