It looks like they are not running normal sessions...only dog ones.
I assume then it's a little way of keeping it popular, in possibly an otherwise quiet month.
As its only dogs, I presume it coincides with a drain and/or big scrub at the end of the month.
I'm not thrilled with the idea of swimming in a pool alongside multiple dogs, or getting into one after hundreds across a weekend has got in. But if it's been completely cleaned ready for when they reopen for humans, it can't be any dirtier that whatever anyone else has got in.