If you were sentenced in Alameda County you could have been sentenced to the "Weekender Program" (Sheriff's Work Alternative Program) which is an alternative to confinement. For each 8-10 hours your work on a weekend day you knock one day off of your sentence. The work is done under the supervision of the Sheriff's Department. However, I think that the sentence cannot exceed 30 days. I do not know whether that means 30 days ACTUAL time one will spend in custody or a 30 day sentence, which will result in less actual time when you are credited with good time/work time credits.
"Weekend jail" in CA generally means that you show up at the jail around 6 p.m. on Friday and get released around 6 p.m. Sunday for as many weekend days as you need to finish your sentence.
I am pretty certain that you were given instructions when you were sentenced about how to enroll in or report to this "weekend jail." Ask your attorney to direct you. If you cannot get a hold of your attorney, get a hold of the county probation department and ask someone there. If you cannot get a satisfactory answer over the phone go down to probation in person. You cannot fool around with this sentence. It is your obligation to pursue the answers to your questions. If you fail to report to "weekend jail" the consequences will be much worse for you.
This answer is for general informational purposes only and not intended to create an attorney-client relationship with the questioner.