It should be spelled out in your decree which day of the week determines if a weekend is 1st, 2nd, etc. My decree states it is Friday that determines which weekend it is, so the FIRST Friday is the FIRST weekend, and so forth. Mother's Day and Father's Day go to that respective parent regardless of which weekend it falls on.
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Some decrees state that weekends are determined by Saturday, but IME, they go by Friday. I'd go back and read your decree for the specific wording. If there is none, I'd google typical custody arrangements for your particular state. In Texas, it goes by Friday as a standard rule.