"Summer ends today."
Translation:El verano acaba hoy.
the article is used if you talk about time or days: example1 : El sabado iré a Barcelona (in english you could say 'saturday i go to barcelona' but better you would say 'on saturday i go to barcelona') example2 : El invierno termina en un mes (in english you could say 'winter ends in a month' but better you would say 'the winter ends in a month')
A common idiom using this verb is ACABAR DE. Por ejemplo: Acaba de salir. (He--or she--just went out.)