Not sure what the event will be...but after a quick google search for holidays for mid May, my best guest would be that it's food related since International Hummus, National Apple Pie, and National Fruit Cocktail Day is on May 13th, National Buttermilk Biscuit Day is on May 14th, and National Chocolate Chips and National Pizza Party Day is on May 15th.
As for my favorite girl of the 3, it would be Fabienne!
Nutaku's Harem Heroes, ID: 41526