The pumpkin of monsters spawned in Halloween does not drop.

Although the pumpkin does not drop without a Looting enchantment, each level of Looting enchantment increase 1% of dropping chance.

**In my mob farm, 15k of wither skeletons is spawned per hour, and 1/4 of them is spawned with pumpkin or jack-o-lantern. Theoretically, there will be about 110 pumpkin and jack-o-lantern dropping, however, none of pumpkin was collected.