Using your email application's calendar.
If you are using Mozilla Thunderbird and would like a calendar like Microsoft Outlook, you need to download an extension called Lightning. It is made by the same people who developed Thunderbird, but kept seperate from the project for various reasons. This add on is easily added through the extensions interface built right into Thunderbird. Alternatively you could download it through your browser.
Start by clicking the Tools menu, then selecting Add-ons.
In the "Get Add-ons" tab, search for "Lightning".
The first result should be "Lightning", with the description beginning with "Manage your daily schedule directly from within Thunderbird."
Click the "Add to Thunderbird" button and accept the install warning.
Wait for the download to finish, and then click the restart button when it pops up in a yellow bar. Alternatively, continue using the application and then close it when you're ready.
The next time Thunderbird opens, the task list will be there on the right hand side of the window. You can then add a new events calendar to use by opening the File menu, selecting New, then clicking Calendar.