These are held in Plaza Ayuntamiento every day at 2pm from the 1st of March until the 19th, the final day of Fallas.
They are incredibly loud and I advise you to go and look one up on You Tube, to experience the volume. By the end of it, the explosions come so thick and fast that the windows in the square shake and if you are close enough, as we were last Saturday, the rumble you get through your whole body is just amazing.
My little video is just the beginning of it, so you don't get the intense sound at the end, but it may give you an idea.
We went to 3 Mascletas during Fallas and they are great; definitely one of my highlights.