* * * * *

        You know, you might want to check the calendar for March 2002

Okay, I've seen this particular meme multiple times now on
GoogleFacePlusBook:

> THIS IS THE ONLY TIME WE WILL SEE AND LIVE THIS EVENT
>
> Calendar for March 2013
>
> Table: March 2013
> Sun   Mon     Tue     Wed     Thr     Fri     Sat>
> ------------------------------
>                                  **1**   **2**>
> **3** 4       5       6       7       **8**   **9**>
> **10**        11      12      13      14      **15**  **16**>
> **17**        18      19      20      21      **22**  **23**>
> **24**        25      26      27      28      **29**  **30**>
> **31**                                            >
>
> This year March has 5 Fridays, 5 Saturdays and 5 Sundays. This happens once
> every 823 years. This is called money bags. So, share this to your friends
> and money will arrive within 4 days. Based on Chinese “Feng Shui.” The one
> who does not share … will be without money.
>

The first time I saw it, I thought, Weird—is that true? Really? 823 years
since the last time? Okay, let's write some code to answer this—which years
has March 1^st fallen on a Friday?

> #!/usr/bin/env lua
>
> function dayofweek(date)
>   local a = math.floor((14 - date.month) / 12)
>   local y = date.year - a
>   local m = date.month + 12 * a - 2
>
>   local d = date.day
>           + y
>           + math.floor(y / 4)
>           - math.floor(y / 100)
>           + math.floor(y / 400)
>           + math.floor(31 * m / 12)
>   return (d % 7) + 1
> end
>
> for year = 1190,2014 do
>   if dayofweek { year = year , month = 3 , day = 1 } == 6 then
>     print(year)
>   end
> end
>

1190 is 823 years ago. Let's see what we get …

> 1191 1196 1202 1213 1219 1224 1230 1241
> 1247 1252 1258 1269 1275 1280 1286 1297
> 1309 1315 1320 1326 1337 1343 1348 1354
> 1365 1371 1376 1382 1393 1399 1405 1411
> 1416 1422 1433 1439 1444 1450 1461 1467
> 1472 1478 1489 1495 1501 1507 1512 1518
> 1529 1535 1540 1546 1557 1563 1568 1574
> 1585 1591 1596 1602 1613 1619 1624 1630
> 1641 1647 1652 1658 1669 1675 1680 1686
> 1697 1709 1715 1720 1726 1737 1743 1748
> 1754 1765 1771 1776 1782 1793 1799 1805
> 1811 1816 1822 1833 1839 1844 1850 1861
> 1867 1872 1878 1889 1895 1901 1907 1912
> 1918 1929 1935 1940 1946 1957 1963 1968
> 1974 1985 1991 1996 2002 2013
>

(okay, I cleaned up the output a bit)

Well … it's happened 118 times since 1190, and it didn't happen 823 years
ago, but 822 years ago, and 817 years ago and … you get the picture.

Hardly a rare occurrence and in thinking about it, any month with 31 days
will have three days that happen five times a month (five Sundays, five
Mondays and five Tuesdays for instance).

Now, I can hope this puts this particularly medium meme (it's not rare, and
it's not well done) out of commission. But alas, I doubt it …


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