Edirne is pleasant most of the year, but spring (April to June) and autumn (September to October) offer the best conditions. Spring brings mild temperatures, wildflowers in the meadows along the Tunca and Meriç rivers, and manageable crowds. Autumn is similarly comfortable and the light is excellent for photography of the mosques.
Summer is hot — July and August regularly reach 35°C — but the city stays functional and the major monuments are cool inside. If your visit coincides with the Kırkpınar oil wrestling festival (held each June or July at Sarayiçi), plan around it: the festival runs for three days and is unlike anything else you'll encounter in Turkey. Book accommodation well in advance if you're coming for Kırkpınar.
Winter is cold and grey, but very quiet. The mosques are uncrowded and the river walks have a melancholy beauty. Edirne is only 230 km from Istanbul and makes an easy day trip or short overnight from the city in any season.