If it is Monday and it becomes 2 a.m., since the light of the sun is no longer visible in the sky then that is the 'Night-of-the-previous-day", so it is 'Monday-Night'. However, one-minute AFTER 'Midnight' is the next "CALENDAR-'DATE", So if Monday is the 3rd of the month then one minute after Midnight is the 4th of the month, but it is STILL ... More @Wikipedia
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