The important thing is when you started it the first day. Start it the first time when the moon is not void. Then on a later day, if the moon is void when you do it, it is not a problem.
A WORK OF MAGIC BEGINS WHEN THE MOON WAS NOT EMPTY, BUT IN THE PLAN OF DAYS THERE IS A DAY WHEN THE EMPTY MOON APPEARS, SHOULD I EXPECT THE MOON TO BE EMPTY? OR I CAN DO IT NORMALLY