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?
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.