Some autistic people have a 'special skill' of being able to tell you what day of the week a particular date lands on. It is one of the autism clichés. But you can find patterns in the calendar that you can use to do this 'trick' quite quickly, even if you don't have an amazing memory or unusually fast mental maths. I thought some people might like to learn how to do it and impress people ;)
(I have tried to write it without maths terminology, although it is a bit simpler if you use a thing called modular arithmetic (click here).)
[A year is a leap year if it's a multiple of 4 but not a multiple of 100, or if it's a multiple of 400. E.g. 1900 isn't, 1904 is, 2000 is because it's 5*400]
If 1st January is a Monday in a particular year, call this year 'special'.
Some of the 'special' years are 1912, 1940, 1968, 1996, 2024... (gaps of 28 years)
(5 years from these) 1917, 1945, 1973, 2001, 2029...
(6 years on again) 1923, 1951, 1979, 2007,2035...
(and 11 years on again) 1934, 1962, 1990, 2018, 2046...
[This pattern will change after 2100 because 2100 is not a leap year, as it's a multiple of 4 and a multiple of 100 but not a multiple of 400.]
Every year, the day of the week of a certain date moves on by 1 day, or 2 days if you go past a leap-day. E.g 12th July is Monday this year so 12th July will be a Tuesday next year.
Example: Day of the week of 15th September 2000
First work out which day 1st January is on. 1st January 2000 is 4 years on from Monday 1st January 1996, and we go past 1 leap day (Feb 29th 1996) so the day of the week moves on by 5 - that's 1 for each year, and 1 extra for the leap day. So it's a Saturday.
Then work out how much the day of the week moves on for the 1st of the month. In a normal year we would have 0,3,3,6,1,4,6,2,5,0,3,5 (for January through to December). If it's a leap year and the date is in March or later, you need to add an extra 1 day. So the 1st Sep 2000 is 5 days moved on from 1st Jan 2000, plus 1 day because of Feb 29th, it's 6 days moved on from 1st Jan 2000.
Then we move on a further 14 days to get from 1st Sep to 15th Sep, which is the same as moving 0 days on because 14 days is 2 whole weeks.
So the answer is Saturday plus 6 days, which is Friday. This is right, you can check it with Google.