Is this the calendar that will be used to schedule every task in the project? And if so, have you already created the enterprise template? Please let us know, as the steps you will need to take will vary, depending on the answers to these questions.
OK, thanks for the quick answers to my questions. Here is what you will need to do:
1. Launch Microsoft Project and connect to Project Online. 2. Click File > New > Enterprise and create a new project from the existing template. 3. Click the Project tab to display the Project ribbon. 4. In the Properties section of the Project ribbon, click the Project Information button. 5. In the Project Information dialog, click the Calendar pick list and select the new enterprise calendar you created, and then click the OK button. This step will force every task to use the new calendar. 6. Apply the Gantt Chart a view, if not already applied., 7. Double-click the Timescale bar at the top of the Gantt Chart pane on the right side of the view. 8. Click the Non-working time tab in the Timescale dialog. 9. Click the Calendar pick list, select the new calendar you just created, and then click the OK button. This step will update the Gantt Chart pane to reflect the schedule shown in the new calendar. 10. Click File > Save As and then click the Save button at the top of the Backstage. 11. In the Save to Project Web App dialog, enter the EXACT SAME NAME as the template you are wanting to edit (you will see the template name at the top of the Save As page, by the way). 12. In the Save to Project Web App dialog, click the Type pick list and select the Template item. 13. In the Custom Fields section of the dialog, specify any custom field settings you may want applied initially in the template. 14. In the Save to Project Web App dialog, click the Save button. 15. In the warning dialog about entering a project name that already exists, click the OK button. 16. In the Save As Template dialog, select all of the checkboxes, and then click the Save button. 17. When the Save job is completed, click File > Close to close the updated template.
I just tested these steps to make sure they work, and I can confirm they do work in my instance of Project Online. Hope this helps.