Tuesday, March 09, 2021
The curious thing about a 56th birthday is that it's always on the same day of the week as the day of the week you were born. No matter whether you were born on a Sunday, Monday, Friday or whatever, your 56th will always be the same. I'm a Tuesday's child, full of grace, and hey presto here I am on my 56th and it's a Tuesday again.
Having a birthday on a Tuesday isn't rare, it happens one seventh of the time. So does having a birthday on the same day as you were born, whatever day that was. What's special is the cast iron guarantee that a 56th birthday brings. It also applies to your 28th birthday, your 84th birthday and possibly your 112th, but we'll get to that.
It is of course all due to leap years.
Normally your birthday skips ahead one day each year because a year is 52-weeks-and-one-day long. If it wasn't for leap years you'd have a birthday on each day of the week every seven years. But a leap year means your birthday skips ahead two days, hence the name, which messes up the pattern. It takes seven leap years for every day of the week to have been skipped once, at which point the cycle starts up again. And seven leap years is 7×4 = 28 years, hence the guarantee that your 28th birthday is on the same day of the week as the day you were born... and every 28 years thereafter.
Here's that pattern graphically, based on my date of birth.
| Mon | Tue | Wed | Thu | Fri | Sat | Sun |
| | 1965 | 1966 | 1967 | | 1968 | 1969 |
| 1970 | 1971 | | 1972 | 1973 | 1974 | 1975 |
| | 1976 | 1977 | 1978 | 1979 | | 1980 |
| 1981 | 1982 | 1983 | | 1984 | 1985 | 1986 |
| 1987 | | 1988 | 1989 | 1990 | 1991 | |
| 1992 | 1993 | 1994 | 1995 | | 1996 | 1997 |
| 1998 | 1999 | | 2000 | 2001 | 2002 | 2003 |
| | 2004 | 2005 | 2006 | 2007 | | 2008 |
| 2009 | 2010 | 2011 | | 2012 | 2013 | 2014 |
| 2015 | | 2016 | 2017 | 2018 | 2019 | |
| 2020 | 2021 | |
Or, if you prefer, here it is with ages.
| Mon | Tue | Wed | Thu | Fri | Sat | Sun |
| | born | 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 | 32 |
| 33 | 34 | | 35 | 36 | 37 | 38 |
| | 39 | 40 | 41 | 42 | | 43 |
| 44 | 45 | 46 | | 47 | 48 | 49 |
| 50 | | 51 | 52 | 53 | 54 | |
| 55 | 56 | |
The underlying 28-year cycle is the same for everyone, but the starting point is different depending on day of birth and how close to a leap year you were born.
The following are always true, no matter what.
» By the time you get to your 28th birthday, every day of the week has come up four times.
» By the time you get to your 56th birthday, every day of the week has come up eight times.
» By the time you get to your 56th birthday, every day of the week has been skipped twice.
» The skipped day moves ahead five days at a time (in my case Fri, Wed, Mon, Sat, Thu, Tue, Sun)
» The gap between birthdays on the same day of the week is normally 6 years (half the time), sometimes 5 years (25% of the time) and sometimes 11 years (25% of the time).
The only exception is for people born on Leap Day, February 29th.
| Mon | Tue | Wed | Thu | Fri | Sat | Sun |
| | 28 | | | | | 32 |
| | | | | 36 | | |
| | | 40 | | | | |
| 44 | | | | | 48 | |
| | | | 52 | | | |
| | 56 | |
They only get a birthday every four years, and this skips ahead five days each time. But every day of the week crops up exactly once over a 28 year cycle, and it's still always true that their 28th and 56th birthdays are on the same day of the week as the day they were born.
There will come a time when the 28 year rule no longer works. End-of-century years are only leap years if they're divisible by 400, which 2000 was but 2100 won't be. This means an eight year gap with no leap years between 2096 and 2104, and that's going to disrupt the pattern spectacularly.
| Mon | Tue | Wed | Thu | Fri | Sat | Sun |
| 2064 | 2065 | 2066 | 2067 | | 2068 | 2069 |
| 2070 | 2071 | | 2072 | 2073 | 2074 | 2075 |
| | 2076 | 2077 | 2078 | 2079 | | 2080 |
| 2081 | 2082 | 2083 | | 2084 | 2085 | 2086 |
| 2087 | | 2088 | 2089 | 2090 | 2091 | |
| 2092 | 2093 | 2094 | 2095 | | 2096 | 2097 |
| 2098 | 2099 | 2100 | 2101 | 2102 | 2103 | |
| 2104 | 2105 | 2106 | 2107 | | 2108 | 2109 |
| 2110 | 2111 | | 2112 | 2113 | 2114 | 2115 |
| | 2116 | 2117 | 2118 | 2119 | | 2120 |
No longer do the columns show a regular four-on/one-off pattern. Instead most days of the week get six consecutive occurrences without skipping, while two days of the week skip twice in close succession. For example the table shows a single Friday birthday between 2090 and 2113, which is a period of 23 years, but also six consecutive unskipped Saturdays. Nothing like that ever happens at present, and hasn't since the turn of the 20th century. Meanwhile those poor souls with a Leap Day birthday will have no day to celebrate between 29th February 2096 and 29th February 2104, which are curiously both Fridays.
2100 not being a leap year is also going to wreck the rule about 28th, 56th, 84th and 112th birthdays. If you're born after 2072 it won't be true that your 28th birthday is on the same day of the week as the day you were born. If you're born after 2044 it won't be true that your 56th birthday is on the same day of the week as the day you were born. If you're born after 2016 it won't be true that your 84th birthday is on the same day of the week as the day you were born. And if you're born after 1988 it won't be true that your 112th birthday is on the same day of the week as the day you were born. Only the last of those might possibly affect anyone reading this.
What I can say for certain is that your 28th, 56th and 84th birthdays are always on the same day of the week as the day you were born... and that this applies to everyone born between 1901 and 2015. I'm looking forward to my Tuesday birthday in 2049 already, fingers crossed.
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