When you have plans or want to get out of the house?!
My two are both asleep, and I swear it's because I wanted to be out of the house at 2pm.
If I wanted to stay in and nap whilst they nap, you know one of them would wake early and the other would refuse to nap!
They must know!
(I know I could wake them; but it's not that important that we went out at 2. Plus I don't want two grumpy kids all afternoon. )