As it says on the tin.

I would like to be able to modify the billing date on my subscription, so that I can bill on a specific day, say the first of every calendar month, Yoshi-Please and thank you.

There is nothing further to elaborate.

I felt so strongly about this that before I made this post, I also called the Square Enix support center(310-846-0345) and provided the same suggestion as feedback to some lady who probably thought I was off me head(I'm joking, she understood why I wanted this and we had a little giggle about personal finance/accounting OCD/Autism).