If you are in Paris the for Easter Week, each day at Notre Dame de Paris is a day of remembrance (in particular Good Friday, when traditionally from 12:30 to 3 pm is the time reserved to remember the slow death of Jesus Christ who has been nailed to a cross and dies ultimately of suffocation). On Good Friday, the relic of the crown of thorns is brought out for veneration.
The Cathedrale de Notre Dame site lists the hours for Easter Sunday services as well as the Saturday night vigil (which I have never seen, but it looks like a fire is lit in front of the cathedral). You may want to check out this Easter Vigil.
Photo from www.cathedaledeparis.com website (See attached link above)
The Sunday service at 10 am is given in Gregorian chant. If you have never heard Gregorian chant, this is a beautiful sound whatever your personal beliefs may be. Music is universal.
The St. Denis Basilica in the northern suburb of St. Denis will also be open on Easter Sunday. Said to be the inspiration for the great Gothic architecture of France under the direction of its abbot Suger, St. Denis is also the final resting place for all but three of the past kings of France.
And if you though that St. Sulpice has been the object of controversy thanks to the De Vinci code, it is truly St. Denis Basilica which has been the subject of some pretty amazing claims even as outrageous as the suggestion that it may have been blessed by Jesus Christ IN THE FLESH?? More on that for another post.