This blog post applies to SharePoint 2010, 2013, and Office 365. This is enterprise, standard or foundation.
This is a cool little trick that Corey Emmons figured out. Since he doesn’t have a blog, I figured I’d at least document it so that you all can see how smart he is. 
The idea is that a task rollup is needed in SharePoint 2010.
(If you happen to have the enterprise version of 2013, then each person does automatically have their own task rollup on their “About Me” site.)
First of all, this is what the end result looks like. A rollup from multiple task lists on different sites. Different task lists have different color codes:
Here’s how to do it:
- Create a calendar, and it doesn’t necessarily have any appointments in it. Mine is called “Test Calendar”. This is something that you may want to create at the root level of your site, so that it’s a central location to see items rolled up from multiple sites.
- Create a task list. Mine is just called “Tasks” and lives at the root level of the site. Create some test tasks in it, and be sure to fill in the start and due dates.
- In “Tasks”, click to create a new view. On the screen where you choose a view format, click Calendar View.
- Name the view “Calendar”. Make it a public view. For time interval, I decided to choose Start Date as my beginning point, and Due Date as my end point. It’s up to you, you can choose any date fields at all.
- Go to (or create) a task list on a different SharePoint site in the same site collection, or a different task list on the same site. On mine, the list is called “Tasks” and it is on a sub site called Demo. Repeat steps 3 and 4.
- Navigate back to that calendar that you created at step 1. Click the Calendar tab in the ribbon.
- Click the Calendars Overlay button.
- On the Calendar Overlay Settings screen, click New Calendar.
- Name it something that indicates the name of that top level task list. I’m just going to call this one Tasks 1. Pick SharePoint for the type of calendar. Pick a color, and then put in the URL of the site where your first task list is. Click Resolve. After you click resolve, the list drop-down will have choices in it. Any type of list or library that you’ve created a calendar view on will be there to pick from. Pick your task list. For the list view, pick Calendar. Check the box to always show, and click OK.
- Repeat steps 8 and 9. For step 9, pick a different color this time. For the Web URL, use the sub site where your other task list is. Click Resolve and pick your sub-site’s task list and Calendar view.
Done! Navigate back up to your calendar. (Mine is called Test Calendar) Now you can see the tasks from both task lists on the calendar, and they span across the days from when each task starts and each one ends. Of course, you don’t have to use the start date and due date, you could just use the due date for both start and end if you want, or any other date field.
Here’s the cool part… you can do this for ANY lists or libraries, as long as you’ve created a calendar view on them!! This works cross-site, within a site collection, but not across different site collections.
Well what do you know! I just found out that Jennifer Mason blogged about pretty much the same thing a while back! SharePoint 2010- Using the Calendar Overlay to Display List Items based on a Category