The recommendation is always to do selective defoliation (leaves that shade the buds) on day 21 after switching to a 12/12 light and dark schedule because the first three weeks of flowering is when the plant reaches its maximum size. After the third week, the plant starts to bulk up. You can still do more pruning later on, but be careful and make sure it’s not too aggressive. The idea of pruning on day 21 is so that the main buds can get more nutrients and the plant can focus its energy on them rather than on branches that receive little light and won’t produce good flowers.