Love in the time of terrorism. That’s the theme of Israel Horovitz’s newest play, “The Day Before Yesterday,’’ which will have a free reading at the Gloucester Stage Company Sunday afternoon. The final play in Horovitz’s Paris trilogy — the others are “My Old Lady’’ and the off-Broadway hit “Out of the Mouths of Babes’’ featuring Estelle Parsons and Judith Ivey — “The Day Before Yesterday’’ follows three couples before, during, and after a rock concert in Paris.