I would guess the 1st & 31st?
If you may have changed plans mid-cycle, it may appear you've been charged 'extra' as you are not refunded for a partial/ unused cycle.
If you believe you are charged double, did you notice an amount in your available funds (which is like an account credit)?
What is the reason PM charges every 30 days and not on fixed date of the month every month?
In case of leap year, whoever signed up on 29 Feb could easily be shifted to 1st Mar.
Although I can't think of any customers would rather have plans be 30 days long, this would simply be a business decision.