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Greetings in the Lord! Thank you for your question.

There is nothing official except that it be the “whole day” (Shorter Catechism 58) which is observed and set apart as holy, which almost certainly means a 24-hour period. In the days of Jesus, for instance, a day was from sunset to sunset. In our day we do not track days in that way, but from midnight to midnight. So, that is how I generally track my Lord’s Day, beginning it at 12:00 AM Sunday morning until midnight that day. But how you track the 24 hours is not as important as the principle that it actually be 24 hours.