The gemara (Rosh Hashanna 29b) rules that blowing a shofar is not forbidden on Rosh Hashanna which falls on Shabbat as a forbidden labor (חכמה ואינה מלאכה). It isn't even rabinically forbidden like musical instruments because it can't be fixed. So why can't I blow a shofar tomorrow to practice?
(NB question asked on a Friday)