The jail's mailroom probably only works Mon. through Fri. at the most. It normally only takes a day for it to make it through the mailroom to the inmate. But if you wrote Wed. night, it probably didn't leave your local post office until sometime Thur. If it's your local jail, your letter probably arrived at the jail on Friday and will get to your boyfriend Monday.
I hope you sent just a regular size envelope and had his jail ID on the envelope. Otherwise, it might be returned to you or take them longer to route it to him.
