I made a bet with myself that there would be web sites dedicated to this...I won..first hit!

here's mine You can do a search on yours, too.

DAY The Celtic and Gaelic word deag or dagh signifies good, excellent, the same as Da, in Welsh. Camden supposes the name to be a contraction of David. Dai, Du, in the Welsh, signifies dark, in allusion to the complexion or color of the hair. Dhu, in Gaelic, the same, dark color, black. Deah, Anglo-Saxon, dark, obscure.