Joyeuses Pâques ! is the right wish for Easter Sunday. You might hear it a few days earlier as well, because people do wish each other Happy Easter in advance.
Pâques is in fact a strange word:
when it is used in singular (and it does keep his ‘s’ anyway, it is said invariable), it is masculine, but becomes a feminine noun in plural. Also, in both cases, it is used without determiner (déterminant):
Pâques* est tombé en mars* cette année (Easter happens to be in March this year). Pâques designates here, the day of Easter.
Je vous souhaite de joyeuses Pâques! (I wish you happy Easter). In this case, Pâques stands for the whole period of Easter celebrations. It is as if we would say: de joyeuses fêtes de Pâques, omitting ‘fêtes’).
In the semaine sainte (Holy Week) you have vendredi saint* (Good Friday) which is not a bank holiday in France, except in Alsace that keeps the German tradition, nor is jeudi saint (Holy Thursday), also called jeudi vert.
* Pâques is usually written with a capital letter, probably as a sign of respect for the religious celebration. On the other hand, mars, vendredi and saint, as any month, days and adjectives, don’t need a capital letter in French.