I have been thinking of the calendar idea for MONTHS now. I was just to lazy to explain my idea.
The calendar would be like a regular one except that the boxes with a C or D check would be filled with a color. Yellow for C, and red for D. If there is a C and D check on the same day, the box would be half yellow and half red. You can hover over the boxes to see which airplane will go into maintenance. The checks are automatically put on the calendar as soon as an a/c arrives. You may wish to change the date but only earlier.
Ex: One of your a/c is due for a C check on April 24, 2002. That is the latest date you can perform the check. You can move it back to let's say...March 25, 2002 because the old date may be too close to another C check. (This would help for financial reasons.)
I don't feel like explaining anymore so I'll let you guys elaborate.