The nearest we have ever got to the August rush was 4th. It was the one and only time we were turned away from Thalassa because they were full and we hadn't booked. Thomas (Vavilas) is now so popular (for good reason) that you must book at least a day ahead all through July and August. But with care you can do whatever you want by planning a bit ahead. (Anathema to me - I flow with the tide when I am in Arillas.)
The beach is NEVER like Brighton (voice of experience) on a bank holiday. We used to live an hour from Brighton and often spent time there. But as Barry says, us lot may find it crowded compared to our usual experiences. My favourite time is very end of October when I have the whole lot to myself, and the sea is like a warm bath.
But I digress. What a wonderful birthday present! 15th August will be very hot, but there will be plenty of space and opportunities for you both.
Phil