SharePoint allows each site to have one or more calendar list. Perhaps every team site in your Farm has a calendar list which is used to record that teams vacations. If you want to then aggregate all of those calendar lists into one place, and show the aggregated content in a calendar, you could use the Lightning Conductor 2010 Web Part.
First, configure the Lightning Conductor Web Part to aggregate all calendar lists within your farm. You could filter by the Vacation Content Type (Custom Content Type) as shown below:
Then, choose the Calendar XSL view.
Click Finish, and you are done!
The Calendar View is built into the Lightning Conductor Web Part already, you will not need to write any XSL. However, If you want to tweak it, and you know XSL, go ahead!
You can download the Lightning Conductor 2010 Web Part from: https://www.lightningtools.com/list-rollup-web-parts.aspx