"Desde marzo hasta mayo."
Translation:From March until May.
That's how Duolingo works - words pop up and you just figure them out :) If you want further explanation, you go research online or here.
From what I've gathered desde .. hasta is closer to since..until while de...a is rather from..to. Seems that the latter can be used for spatial relationships while the former can't, e.g. De Madrid a Bilbao.