I hope this message finds you well, present circumstances excluded. I am a licensed attorney with over a dozen years of practice experience. It is a pleasure to assist you today.
The fact that it specifically mentions "one-day holiday" leads me to believe that the intention of the court (though they worded this ambiguously) would be for the mother to get your daughter on Memorial Day. More specifically, the if we borrow from the mechanism of delivery, she would probably get your daughter on 6 pm Sunday and have her for Memorial Day as well.
Understand that this is ambiguous language though...meaning not cut and dry. I base my interpretation on the mention of "one-day" as opposed to more specific language like "holiday weekend". That said, her argument would be that Memorial Day and Labor Day are one day holidays that always take place on a Monday. Therefore, she could argue that she is supposed to get her for the entire weekend.
My advice is to attempt to be as amicable as possible. I would offer for her to have her the entire weekend, if she is willing to trade out weekends next weekend. If she is unwilling, then I think you have a decent argument to return your daughter Sunday night for Memorial Day the next day.
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Best wishes going forward!