Irrational? Absolutely. With calendars built into our cell phones and PCs, there's pretty much no reason to ever carry a paper calendar like this one. But it's so cool.
It's not like a standard calendar -- it doesn't lay out 12 sequential months, so you can see what day Arbor Day falls on this year. Instead, you'll see a typical 31 day grid bordered by 12 stripes, one for each month. Find the day you want in a particular month, like Mondays in June, and then run your finger down to see what dates fall on that day. Answer: 7, 14, 21, and 28.
Or, go the other way: What day is the 14 of July this year? Wednesday.