The day of full moon and the day before are ‘flower days’ according to Maria Thun’s calendar. Two days before, when Charles reckons is the best day to sow, is a root day. So great for carrots, beetroot and the like.
I sowed Marigolds one day before the full moon in April on a flower day and they did great, so if I were you, I would sow your flowers on 20th May if you can or, if not, on the 19th or 21st.