Is is okay to have my sons hair cut the day before his birthday? He is 11 months now and will be turning a year old on the 15th of september

That should be okay. For me, there's no need to wait for the actual birthday to have the baby's hair cut just like what most superstitious beliefs say. It would also be in time for your baby's first birthday party. I bet you want all of your babies to look neat on his big day.