Happy Memorial Day Weekend!

I have tried googling this, but to no avail:

My 18 month old is completely* inconsistent with how he reacts to bedtime, even though I do everything the same.

For a week in a row, he will go upstairs cheerfully, read a handful of books, walk over to his crib and put himself to sleep without a peep.

Then for three nights he will WAIL and I mean WAIL at having to get in his crib. No fever, no difference in naps, no difference in routine or bedtime.

Then back to normal.

ETA: He sleeps overnight (about 11 hours) with no wake-ups about half the time. The other half of the time he wakes 1-3 times and I will go up and at him back to sleep. He is a terrific napper, though, never resisting and often sleeping 2.5 hours.

What could be causing his different attitude with respect to bedtime?