Well, that's an easy one. It has to be Red Campion
In the last 2 weeks of April (before, if it's hot but not dry), it throws up flower spikes from the basal rosette. These open slowly, over the course of 7 days. But one, really hot April day can bring them out.
And they last - until July. Sometimes flowering again in Autumn.
Red Campion loves some shade; it is a plant of woodland, woodland edge, and shady corners.
That said, we've got it flowering now, in most of our meadow projects locally, and these are in full sun. Long grass helps to shield them from the sun later in the season, and they thrive.