Is my design too simple?

It's cutting it close. I generally like simple, spare designs but many people don't. One problem I think are the typestyles. I think a sans-serif font would look better (Arial, Helvetica, Tahoma, Verdana, Trebuchet, etc.).

I think having the 'ORGanimal is' below the fold could be an issue.

I kinda got that it was a calendar for pets, but wasn't sure exactly what the deal was.

I don't think your design is too simple. But I think you might want to take a closer look at the flow. What people see and how they understand what they are supposed to do.