Using your words I'd say "Sit sabbatum beatum" which would mean "May (your) Sabbath be happy."
I like your use of beatus-- it is used in a divine context fairly often, and it contains the positive sentiment that this phase intends.
These two are off-the-cuff guesses at potential ways to achieve your meaning.
You could say (singular/plural) "Gaude/Gaudete, nam est Sabbatum!" Rejoice, for it is the Sabbath!
Or maybe "Gaude/Gaudete cum Sabbatum (sit/veniat/oriatur/adeat/succedat)" I left the verb open-ended since the goal is to emphasize the HAPPY and the SABBATH portions of the phrase. Having a verb in the cum clause seemed unnecessary. I'd translate this into English as "Rejoice because of the Sabbath," though including a verb could change some nuances of the translation.