There are very few bad times to say comon. Comon can mean excited, lit, hype. It can also be used as a greeting similar to "yo" or "whats up gangsta". Comon is similar to pirates saying arg. Normally the word comon is used in a positively comical way depending on situation meanings can vary.
ex/ "yo I fucked Jessica last night"-Tom "oh yea she's hot, comon"- Jeff
ex/" I just bought a case and bottle for the beach tomorrow... COMON"
ex/ *Wu Tangs a blunt* "aye comon"