I'd think very carefully about how you want to get the information out.
What reports are people going to want? (go and ask them)
Ask them for the business rules that apply.
Model that data - put in spoof data and make sure you get get the data out without nested loops of SQL statements.
Show the people how the reports would look and behave, check that is what they want, and listen carefully.
Decide what SQL selects might be invoked if someone wanted to book a holiday, and the max number of people being allowed off that week had been reached.
Putting the info in is usually the easy part and can be meddled with later to make it easier etc, but making sure you can get the reports out efficiently means getting the right fields lined up, with the right type of data -- work your way out from that. Is my advice.