They don't happen on the same day.Some traders are on vacation next week. Volatility is a double edged sword. The Fed and Brexit both happen on Wednesday. One bad trade and you may lose a lot. Another reason why investors who've been trading leveraged instruments for many years do well when volatility is low and markets aren't on a roller coaster ride.
Brexit/EU Referendum voting starts on Thursday 23rd and polls close at 22:00 BST.
Apparently there are no exit polls(?) so the first results are expected to come in from ~23:30 on the Thursday and will carry on coming until ~07:00 on Friday 24th June, with a half-decent projection of the result probably due around 04:00 on Friday 24th June.