When to plant Onion sets in District of Columbia

In spring, Feb 15 – Mar 20 in Washington.

In fall, Oct 1 – Dec 1 in Washington.

USDA's earliest safe spring and latest safe fall planting dates, matched to each place's average frost dates.

Onion sets planting dates by place in District of Columbia
PlaceSpringFall
WashingtonFeb 15 – Mar 20Oct 1 – Dec 1Now

Planting dates: USDA Home and Garden Bulletin No. 9 (1967), tables 4 and 5. Frost dates: NOAA U.S. Climate Normals 1991–2020. “—” means the place's frost date is outside the table. The full USDA table for onion sets.