I guess it has to do with it being the summer, out of all the seasons and not just a summer.

Anyways, most European languages do this, but remember that English often is the odd one out.

Danish: Sommeren er forbi.

German: Der Sommer ist vorbei.

Esperanto: La somero pasis.

Dutch: De zomer is voorbij.

Spanish: El verano terminó.

Icelandic: Sumarið er búið.

Swedish: Sommaren är över.

Norwegian: Sommeren er over.

Duo now accepts the sentence without "La" before "somero."