Friday the Threeteenth?

Why, other than because of general English perversity, do we say "third" and "thirteen" rather than "threed" (or even "threeth") and "threeteen"?

This is an example of what is called metathesis, where speakers switch around the order of sounds in a word. You'll be wanting to tell all your friends about this, so you should know that metathesis is pronounced "muh TATH uh sis". Metathesis is how "asterisk" becomes "asterix" and "pretty" becomes "purty", but before you get all huffy about how you would never be so sloppy in your pronunciation, consider that the word "wasp" was originally "waps". 

So, back to "three". The ordinal number was indeed "thrid" (pronounced "threed") in Old English and indeed was the prevalent form till the 1500s. But up in Northumberland, a metathesized form, "third", cropped up as early as the 950s and gradually made its way south till it established itself as the standard form. This apparently also affected "threeteen", which was the only form until the 1400s, but by 1700 had entirely lost out to the metathesized upstart. 

For an explanation of why the ordinal of "one" is "first", click here 
For the story on "second", see this post: 
http://katherinebarber.blogspot.ca/2012/11/friday-twoth.html
 

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