This reading is an exception. Let's use this mnemonic to remember it:
The day before last you bought an "O" toe toy (おととい). It's a kid's toy in the shape of the letter O that you put on your toe. You know it was the day before last because you wore it all yesterday, and now today your toe is sore. If only you could go back to the day before yesterday to tell yourself not to buy that O toe toy! Or at least not to wear it for the whole next day!
It's a bit late, but the day before yesterday, May 5, was Luffy's birthday.
I was available the day before last, but this month all of my other days are entirely full already.
The day before last I caught my husband cheating on me.