# Happy Odd Day 5/7/9

Well, just as we’ve gotten our strength back from the revelries of Square Root Day (3/3/09) and Pi Day (3/14), its time to celebrate the next fabulous numerical holiday:  Odd Day.  Now, lots of dates are made up of odd numbers, but it can’t be an Odd Day unless the date consists of consecutive odds.  That means, of course, that the next one won’t come along  until July 9, 2011.   So get out and celebrate in some odd way!!

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## 8 thoughts on “Happy Odd Day 5/7/9”

1. Certainly, every date must have some special property no other date has. We will prove this by contradiction:

Assume there are some dates which have no special property. Construct the set S of such dates. Assuming physicists are correct that the universe began a finite amount of time ago, then this set is wellordered under the relation that previous dates are less than future dates. Find the least date in this set i.e. the first day with no special property. But then that day would have the property that it is the first day with no other special property, and hence would not be in S. By contradiction, S must be the empty set.

So if we want to be picky, every date has something special about it.

• Logan,

Your proof reminds me of this story from Stephen Hawking’s “Brief History of Time”:

A well-known scientist (some say it was Bertrand Russell) once gave a public lecture on astronomy. He described how the earth orbits around the sun and how the sun, in turn, orbits around the center of a vast collection of stars called our galaxy. At the end of the lecture, a little old lady at the back of the room got up and said: “What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise.” The scientist gave a superior smile before replying, “What is the tortoise standing on?” “You’re very clever, young man, very clever,” said the old lady. “But it’s turtles all the way down!”

2. When I first read this post, I agreed with many others thoughts of, “how many special math days can there be? Surely you can come up with something for everyday.” But then I read one of the related blogs linked above where they said that odd day only happens six times in a century! Now, odd day feels a little more special! Six seems a little too few however… I may have to do some further research on this “fact”.

• well, to be an odd day, the month has to be 1, 3, 5, 7, 9, or 11. Since it requires consecutive odds, the 6 odd days in a century are 1/3/xx05, 3/5/xx07, 5/7/xx09, 7/9/xx11, 9/11/xx13, and 11/13/xx15. Once the year get past 15, there will be no month that can make it an odd day. So odd days are indeed rare.

The proof is pure genius, I like it very much.

3. I think it’s also interesting that they are all composed in the first 15 years of every century. We have to find some special days for the other 85 years!