I know someone born on the 13th and we were talking about how Friday the 13th is his birthday from time to time. I thought that might be a good question for Wolfram|Alpha but no matter how I phrase it I can't get it right.

I've (unsuccessfully) tried

  • Fridays which are the 13th of June
  • Next 100 Fridays and are in June
  • List of Friday the 13ths
  • List of days
  • Today is Friday?

What's (kinda) worked

  • First Friday next month (correct)
  • 13th next month (Interpreted as "Thursday next month"
  • First Friday in next year (correct)
  • 13th day of this month (correct)

Based on that working entries, I tired these, none of which worked:

  • {Each, Every, All, List of} Friday in next year
  • (13th in next month) and (Friday)

1 Answer 1


You can compute it using a perpetual calenda r http://en.wikipedia.org/wiki/Perpetual_calendar : basically, re the issue of what day falls in which date, there are , for our Western calendar, just 14 possible arrangements of day vs. date: the 1st day of the year can be either Monday, Tuesday,..., Sunday, and the year can be a leap year or non-leap year. Once you know what day January 1st fell on your frinds birth year, you can figure it out.

  • $\begingroup$ It's easy enough to find out which day of the week the 1st of his birth year is just by looking it up. Same with leap year. Can Wolfram|Alpha be asked something like "Show all years where January 1st is a ___day AND the year is [not] a leap year"? $\endgroup$
    – Huckle
    Jun 29, 2014 at 21:49
  • $\begingroup$ Ah, sorry, missed the Wolfram part. Are you married to the idea of Wolfram, or would you consider other ways of figuring out the day-date? You may, e.g., use the list of perpetual calendars. Basically, you figure out which number is your friend's calendar on the list of perpetual ones, and then figure out the rest from there. $\endgroup$
    – user99680
    Jun 29, 2014 at 21:53
  • $\begingroup$ I could write a program to do it in about 30 seconds. This was really about figuring out how to get Wolfram|Alpha to do something useful (which short of calculus I can never seem to get it to do). $\endgroup$
    – Huckle
    Jun 29, 2014 at 21:58
  • $\begingroup$ I see, let me give it a try. $\endgroup$
    – user99680
    Jun 29, 2014 at 22:00
  • $\begingroup$ "January 1st day of the week" and "Next January 1st day of the week" worked for me, but no other modifiers like "list of", "next 10", "set of", etc. work. $\endgroup$
    – Huckle
    Jun 29, 2014 at 22:01

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