# Top 10 calendar facts

Selected as the most interesting among mathematics related questions in
Augustus was born on 1 January 10BC. Which birthday did he celebrate at 1 January 10AD?
19. There was no year number zero. There were only 1999 years between 1000BC and 1000AD.
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5
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Which day was the last of the 20th century?
31 Dec 2000. There was no year number zero. So, the first century started on 1 Jan 1. Similarly, 21th century started 1 Jan 2001.
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3
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How many times in a year may Friday 13th happen?
Up to 3. Friday 13th happens three times in a year only if it starts on Thursday or (leap year) on Sunday.
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If there is Friday 13th in a month, then ...
This month starts on Sunday. If 1st is Sunday then Friday is on 6th and 13th.
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1
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How many months have 31 days in them?
7. January, March, May, July, August, October and December.
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