As flowers fade and tomatoes droop on the vine, another sign of summer's waning appears - the last free outdoor concerts. Here's a wrap-up for this weekend.

The Singing Sergeants don't even pause a half-rest of autumn. Friday at 8 at the Jefferson Memorial they'll present a "Christmas in August" program.

On Saturday, a festival of Latin American dances and music takes over Gallery Place (near Seventh and F Streets NW) from 1:30 to 7. (Call 673-7635.) That evening, Ramsey Lewis plays in Fort Dupont Park at 8:30. (Call 426-7723.)

Sunday offers at least a half-dozen possibilities. There's jazz at Glen Echo Park: Bill Harris at 1, and Tim Eyerman and East Coast Offering, 3:30. (Call 492-6282.) On the Canal at 30th Street NW, the Richard Smallwood Singers and the Federal Jazz Commission play 1:30 to 4:30. Out at Bull Run Regional Park in Centreville, you can hear the Santmyer Family perform country-western music at 2. (Call 631-0550.) From 3 to 7, Appalachian Rain, a local ensemble, does bluegrass in Annandale's Wakefield Park. (Call 941-5006). The National Concert Band of America plays at 6:30 in Meadow Pond Amphithester, Burke Center. (Call 978-7940). And, last but not least, (thanks, stalwart music lovers and cheapskates, for sticking with us), the Marine Band at 8:30, by the Jefferson Memorial.