The dog weekends don't start until mid-September. (Sep 15.) The reason it runs for three weeks right then is because they want to catch the milder weather to make sure people come, because Dogtember is a really, really important month for the Lido, which is a charity. They raise more money from those few weekends than all the previous season's swimming, about 250k. If it got too cold, and rainy, the event would be less popular and they wouldn't get the same money.
It's almost as if they thought it all through!!!!
Plus, the doggies are people too and they deserve a nice swim and some doggie ice cream.