It can take a couple of days from when your solicitor formally requests the funds, to the transfer happening. Depending on how many final checks happen on the bank side.
Your solicitor can check with the bank in advance. What I usually see happen is if you wanted to close on Friday, the request would go in on the Monday or Tuesday. The bank can issue the funds so that they're in your solicitors account first thing Friday morning, ready for them to execute a same day transfer to the vendors solicitor. If the funds are issued early, you will incur an interest charge on it.