11 Dec '23 13:05

@venda said
I go for a pre xmas drink with a friend on a Friday or Saturday night in December.
We have done this every year since 2012.
What are the chances on us repeating the evening on the same day and date this year?
I realise we have already missed 2 dates

So we have 24 possible dates (1st to 24th).

You already have (maximum of) 9 dates were you performed your ritual.

So you have a probability of 9/24 to hit on the same date (or less if you hit on the same date before).

The date is on the same weekday every seven years (if one leap year is in between) or after six years (if there have been two). So we had one leap year to hit the same weekday in 2016. And two more to hit 2011. So we have to assume that you performed the ritual 2016, otherwise probability is zero.

So now we have one single date that you can hit to find the same date with the same weekday. Since you give two weekdays, probability is 1/8.

Posers and Puzzles

Probability question