On the summer solstice, 21st June at solar noon, the sun at its zenith enters the basilica through the southern windows and draws a path of light on the ground in the central axis of the nave; nine perfectly aligned patches of light lead from the narthex to the choir. On winter solstice, 21st December at solar noon, the sun hits the capitals of the upper parts of the nave looking south in perfect alignment.
Year round with La Maison du Visiteur; on Sundays from Easter to 1st November, and daily in July and August with fraternité monastique de Jérusalem. You can also hire an audio-guide from the Tourist Information Office
Check the timetable on the web site fraternité monastique