A stunt masquerading as a statement. In Day Night Day Night, Julia Loktev's maddeningly austere and self-important drama, an American woman (Luisa Williams), 19 years old, arrives in Manhattan to carry out a mission as a suicide bomber. Is she a homegrown Islamic terrorist? An especially irate lefty blogger? We're never given a clue, so the whole It can happen here! scenario unfolds in a vague ''cautionary'' vacuum. The silent preparatory rituals meetings with hooded comrades, the strapping on of the bomb culminate with a death walk, in real time, through Times Square, a sequence of the most purely opportunistic suspense.