According to The Daily Globe World Dates Archive, April is (amongst other things):
Grass Month
Uh-huh Month
Both National Anxiety Month, and Stress Awareness Month
Not just National Humor Month, but also Holy Humor Month
and my favorite,
International Twit Award Month
April also contains:
Cruelest Month Week
Straw Hat Week
Egg Salad Week
Lefty Awareness Week
National Bubblegum Week
Publicity Stunt Week, which is no doubt somewhat undermined by TV Turn-Off Week and National TV-Free Week
Both National Library Week and Library Forgiveness Week, which should prepare you for National Reading a Road Map Week
National Lingerie Week, which I'm sure certain of our number would like to combine with National Give-A-Sample Week
I'm almost afraid to look at May...
Other "This Day In History" sites:
YarraNet
Those Were The Days
History Channel
Any Day In History