The SharePoint Calendar Content Type includes an “All Day Event” Yes/No Checkbox Field. By default, this value is set to false. If you want to default this value to True, you have to use a little jQuery in your NewForm.aspx. Perform the following steps to default the All Day Event Field to True:

1. Go to your Calendar List. Select the “List” Tab
2. Click the “Form Web Parts” drop down and select the “New Form”
3. Add a Content Editor Webpart to the page.
4. Download the latest jQuery.Min file and upload to the Site Assets folder on your SharePoint.