John Gill Commentary Judges 14:12

"And Samson said unto them, Let me now put forth a riddle unto you: if ye can declare it unto me within the seven days of the feast, and find it out, then I will give you thirty linen garments and thirty changes of raiment;" — Judges 14:12 (ASV)

And Samson said to them
His thirty companions, very likely on the first day of the feast:

I will now put forth a riddle to you :
a secret, hidden, abstruse thing, not easy to be understood; a dark saying, wrapped up in figurative terms; and this he proposed as an amusement to them, to exercise their wits, which it seems was usual to entertain guests with, and might be both pleasing and profitable:

if you can certainly declare it to me within the seven days of the
feast ;
for so long the nuptial feast was usually kept, see (Genesis 29:27Genesis 29:28) . If they could find it out; and with clearness and certainty explain the riddle to him within that period of time, which was giving them time enough to do it in:

then I will give you thirty sheets, and thirty change of garments :
that is, every man one of each. By "sheets" he means, as Kimchi and Ben Melech interpret it, a covering of the body in the night next to the flesh, in which a man lies, and was made of linen; meaning either what we call shirts, or bed sheet, and by change of raiment, a suit of clothes worn in the daytime.