If you are in Mainz during Christmas time, please visit the Christmas markets. Generous choice of great food and gifts. We love their Mushroom dish and also the German Sausages ! We can never get enough of it !
Also try the Gruwine (Warm Mulled Wine). This promise to warm you up in the freezing winter.
Fondest memory: This is how Christmas should be celebrated !
Every July the swr3 open air music festival takes place in the "Regierungsviertel" of Mainz. It usually starts in the early afternoon and ends around midnight. The entry is free so it is always pretty crowded!
The bands performing in the afternoon are usually the not so well-known ones, towards the evening more well-konwn as well as popular bands perfom.
As far as I know you're not allowed to bring your own drinks etc. to the festival but there are lots of drink and food stalls, so no worries ;-)
Johannisfest takes place every year in the end of June. It lasts for four days and ends with huge fireworks on the Rhine River.
During the festival the Rhine River promenade as well as the city centre turn into a fair. Also, several radio stations of Rheinland-Pfalz are usually present.