Weißer Sonntag is not to be mixed up with Whitsunday (Pentecost, Pfingsten). It is the first Sunday after Easter when the catholic parishes traditionally celebrate the first Holy Communion of the nine year olds. After a year of lessons the children are blessed and admitted to communion for the first time during a festive mass. As they are dressed up in white robes for this occasion, the day has been named the "white" Sunday.
A kid's Kommunion requires a family reunion and a festive lunch after church. In case you plan to visit a restaurant that day with a larger party, make a reservation well in advance - 40 Kommunionkinder in a parish mean 40 families booking all better restaurants in the surroundings solid.
The Protestant churches have their confirmation celebrations in spring, too, but dates vary.