Fella joins the navy, they're at sea for a week and he says to his shipmate, " jeez I'm so horny ".
His mate tells him about a barrel below decks with a bung hole in it and told him to go pleasure himself there.
He does this and next day tells his mate that it was great and asks how many days he can do this.
His mate says every day except Wednesday.
He asks " Why not on Wednesdays ".
His mate told him that it was his turn inside the barrel on Wednesdays.