Yours, Etc.: Origins and Uses of 8 Sign-Offs
1897, American English, thought to be an alteration of Sunday, perhaps re-spelled in deference to religious feelings; but the reason for the name is uncertain; perhaps "ice cream left over from Sunday, on sale later." For a fuller account of the speculations, see H.L. Mencken, "The American Language," Supplement I (1945), pp.376-7.