As Richard said relying on when the post office is open/closed is a good rule of thumb but it does not always hold true. There are 4 holidays designated by a specific date: New Year's Day, Independence Day, Veterans' Day and Christmas Day.
If any of these 4 holidays fall on a Sunday and the post office observes the holiday either on the preceding Saturday or the following Monday those days are still rescission days even though the post office is closed.
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The opinions expressed are mine and they are not to be taken as legal advice.