How should tenants who rent weekly/bi-weekly pay their rent?

I rent the first floor of my house by rooms. There is no lease; I give tenants the choice of paying weekly or bi-weekly, but not monthly. I have so far encountered two tenants who argue that they do not owe rent for the fifth week of months that carry five weeks because a month typically has only four weeks and they would be paying extra if they do. My argument is that whether there are generally four weeks in a month, they are renting weekly/bi-weekly and should not have a week for free. I cannot say to the bank that I'm short on my mortgage because a particular month has a particular amount of days. I understand for tenants who are on a month to month lease the monthly rent is always the same. Am I right or wrong in my logic?

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