To avoid conflicts in planning events,where can I find a calendar of national events, sports, holidays, etc?

I often have to plan office and company events, but I never know if there is a Superbowl, March madness, religious holiday,, etc. with which I might have a conflict. Is there one source where I can look to see what major events are scheduled weeks or months in advance so I can plan better? Obvioius sources for holidays are easy, but knowing what day major sports events are in two months, etc is a real problem. Any professional events planners out there.

To avoid conflicts in planning events,where can I find a calendar of national events, sports, holidays, etc?
Well, seems like you would want an "international" calendar to be aware of the religious events as their souls report to lands afar. You have to listen to the radio and tv to figure out when the superbowl is. Don't give them much leeway or credit if they ditch the events in favor of calendar excuses. That way you can seperate the "wheat from the chaff" and keep the chaff.