The History of the Letter Y in the Roman Alphabet

The word day was actually spelled D-A-G in Old English. The key point here is that Old English retained that G on the end of the word since it had once represented a G sound, just like in modern German. But the French influence scribes took care of all of that after 1066. Those Gs were removed, and the French Y was put in their place. And day finally got its modern spelling, D-a-Y.