I would definitively choose one of the weekdays, we went last year that time and it wasn't busy at all as no French or UK school holidays... weekend always busy plus with 25th anniversary, if possible always try to go during weekday...
My family will be there the 6th/7th. We decided to do Alsace over weekend and then head there Tuesday morning and leave Wednesday night for London. Hoping it isn't crowded and we can both get everything done and soak in the atmosphere in less than 48 hours. And I use "everything" loosely...