You might want to check out Is there an easy way to get the start date of the current week?. Although it's not exactly what you're trying to do, it may be close enough that the suggestions in that thread might be able to help you out.

In reply to Re: Calculating latest 'Sunday' date on the first of the month by dasgar
in thread Calculating latest 'Sunday' date on the first of the month by Cicatrix

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