An appeal to the public to mail Rosh Hashonah greeting cards on or before September 19 so as to avoid congestion and delay was made yesterday by Postmaster John J. Kiely. Early mailing, he emphasized, will permit Jewish post office employes to observe the holidays. Careful addressing, prepayment of proper postage and the use of envelopes no smaller than 2Â¾ by 4 inches are other suggestions made. Rosh Hashonah starts at sundown, September 22, and ends at sundown, September 24. Yom Kippur begins at sunset, October 1, and ends at sunset, October 2.