People sometimes write something like "let's finish this work by Saturday night" or "let's meet on Saturday night". Which day and time do they mean? Is it short for tonight, and then it's on Saturday somewhere between 6pm and 11:59pm, just before Sunday starts? Or is it after midnight, which is after Friday evening, when Saturday just starts?

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    Technically, after midnight is early Saturday morning, not Saturday night, although some people might refer to those wee hours of the morning as a continuation of Friday night. (I can say, "Last night, we went to the concert, and then I crashed at Tony house," even if we didn't get to Tony's until 1 o'clock in the morning). But this question is so basic I'm afraid I'll have to vote to close – people simply don't refer to after midnight Friday as "Saturday night." – J.R. Nov 3 '12 at 8:36
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    Saturday night comes between Saturday and Sunday. – user16269 Nov 3 '12 at 8:40
  • Who told you that “night” begins at 6pm? That would be unusual, for twilight to end at 6pm exactly. The sun is rarely so punctual. Also, when people talk about daytime and nighttime, they pay no heed to this 11:59 thing. – tchrist Nov 3 '12 at 8:59
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    @tchrist: In some contexts, 6PM is indeed the boundary between "afternoon" and "evening". (I believe TV listings use that convention, for example.) That said, I agree with you, 6PM is hardly an official start of "night" in all contexts. – J.R. Nov 3 '12 at 9:40
  • Re closing comment " It can be definitively and permanently answered by a single link to a standard internet reference source designed specifically to find that type of information." No. It can not. It's one of those "tricky little question" that does NOT have an exact answer (see my answer) and is worth exploring philosophically. – Russell McMahon Nov 4 '12 at 10:56

This is trickier than most admit. Some answers say Saturday night stops at midnight BUT do not properly consider the early hours of Saturday.

"Saturday night" when used without special explanation or if not in an understood special context will be taken to mean the period from approximately sunset onwards on Saturday. Whether it stops at midnight depends on context.

If you are only referring to a period of time somewhere between midnight and dawn on Saturday you would definitely call it "Saturday morning" or early morning or similar. So too you'd call the 1st 6 hours or so "Sunday morning".
BUT if you worked from 7pm Saturday to say 6am Sunday you might say "I worked all Saturday night", or "I worked all Saturday night- right through to morning".

You could say "... Sunday morning..." in that context but if you did not, nobody would feel you had said something wrong and all would understand what you meant.

If you are Orthodox Jewish then each day ends at 6pm, so Sunday begins at 6pm on week-day-Saturday. How they define "Saturday night" is unknown to me. Properly there is no night at the end of Saturday so arguably all the night period is at the start of the day, so "Saturday night" starts at week-day-Friday 6pm.

In dealing with the Jewish holy day of Saturday, Wikipedia - Shabbat says

  • According to halakha, Shabbat is observed from a few minutes before sunset on Friday evening until the appearance of three stars in the sky on Saturday night.[1]

Saturday night is the night before Sunday or Sunday morning. So if you have already entered Saturday, you could say tonight is Saturday night, which is the start of Sunday.

  • "let's finish this work by Saturday night" would be some time between the time said and midnight Saturday.

  • "let's meet on Saturday night" would be after dusk and before midnight Saturday.

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    I don't agree with the implied midnight deadline; certainly not in all cases. (I could see finishing a project at 12:45 AM, and thinking I had accomplished my goal – only a pedant would argue that we hadn't completed the work "by Saturday night" simply because our efforts extended past midnight). "By Saturday night" can mean "before we go to bed on Saturday" as easily as "before midnight on Saturday." – J.R. Nov 3 '12 at 9:36

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