Overlay multiple calendars in SharePoint to create a master calendar. This feature only works when your calendars are on the same site
Your team can now have a master calendar that includes calendar rollup information from calendars that are nestled deeper into the SharePoint site hierarchy. You can overlay up to 10 calendars over one another to create this master calendar, and each calendar is displayed in a different color code. (It’s kind of like what happens when you link a SharePoint calendar to Outlook and then overlay the two calendars within the Outlook client.)
At the main SharePoint site where you want the master calendar to live, either create a new calendar list from the Site Actions menu, or use an existing calendar that already is configured at the level you want. On the Calendar tab in the Ribbon, click Calendars Overlay, and then click the New Calendar link to add a new calendar to the overlay view.
This next part is a bit trickier: You have to give SharePoint some information about the calendar that you are adding, such as a name and its plain-English description, and you need to have the URL of the site that houses the calendar. This is the link to the site itself, not a link directly to the calendar. After you enter in the web URL and click the Resolve button, SharePoint will use that URL to crawl the site and find the calendar lists and views.
If the site you provided has many calendar lists or views, SharePoint will ask you to choose the ones you want to overlay; just select the box next to Always Show, and when you’re done, click OK. Lather, rinse, repeat for each separate website (again, up to 10 individual calendars) that contains calendars you want to overlay, and then click OK. Now you’ll see all of those calendars rolled up into one master calendar.