When checking in for your first flight see if the bags can be tagged all the way to your final destination even though you are on two different trips. If not, I would find a way not to check a bag (travel lighter and/or buy tioletries upon your arrival at your destination.) Like Analise has said, LGA can be a zoo in the summer time and it would appear that the 5:55PM flight is likely the last one out that day. Also, if you miss that flight you could be looking a change fee and reprice of the ticket.