My 5 month old wakes every night at 9.30pm. Why?!
This could be either over tiredness or under tiredness!
A little trick to help you decide is, how long it takes your baby to fall asleep at bedtime. If it is less than 15 minutes your baby is probably over tired so I would bring bedtime forward.
If it is more then 15-20 minutes then I would lengthen the time between the last nap and bedtime. It could be that not enough sleep pressure has built up during the last wakeful window.