Henry Ernest Dudeney/Modern Puzzles/219 - A Calendar Puzzle

Modern Puzzles by Henry Ernest Dudeney: $219$

A Calendar Puzzle

I have stated in my book, Amusements in Mathematics, that, under our present calendar rules,

the first day of a century cannot fall on a Sunday or a Wednesday or a Friday.

I am frequently asked the reason why.

Try to explain the mystery in as simple a way as possible.

Note that $1$st January $1901$ was the first day of the $20$th century, not $1900$.

Sources

• 1926: Henry Ernest Dudeney: Modern Puzzles ... (previous) ... (next): Unclassified Problems: $219$. -- A Calendar Puzzle
• 1968: Henry Ernest Dudeney: 536 Puzzles & Curious Problems ... (previous) ... (next): Unclassified Puzzles: $516$. A Calendar Puzzle