Should I buy the airline tickets now?

My cousin is having his bachelor party down in Miami at the end of May. My bro would like to attend too however he’s starting a new job in mid-March. He could not book his own ticket due to financial restraints. So do I book our tickets now, hoping be can get the needed days off once May comes around? If he can’t, how are airline return policies? Would it a big deal to ask we take a Saturday morning flight instead of a Friday afternoon flight?