By our calculations, you should look at sowing Buttercup about 21 days before your last frost date.
Transplant seedlings when about 3 weeks old; seedlings often do not take well to transplantation so growing in a peat/newspaper pot may work best.
Ensure that temperatures are mild and all chance of frost has passed before planting out, as Buttercup is a tender plant.By our calculations, you should look at planting out Buttercup about 7 days after your last frost date.