No specific time. If it's a parish-wide activity, it's generally done on a weekend. School May crownings may take place any week day. And in some schools, this happens daily, with a different child every day bringing a flower crown.
At the convent school I attended, we had something called Mary Day. We were allowed to be out of uniform and all wore lovely spring dresses, and ribbons in our hair. We processed around the block our school occupied, saying the rosary. Then we all gathered by the life-sized statue of the Virgin Mary, and the sodality prefect crowned the statue with a wreath of flowers while we sang a special hymn devoted to the May crowning.