As many of you must've noticed, Mozilla's email client Thunderbird (unlike Outlook) doesn't come with a default Calendar Extension. So, if you want to keep a schedule, organize meetings etc. you have to Add an extension. Though there are a lot of options but I suggest using the Lightning tool. You can install it via following steps:

1. Go to the Options menu and click on Add-ons
2. In the search box, type Lightning (the current version is 3.3.3)
3. Click on the Install button and you're good to go

It should appear as a docked pane to the right of your Thunderbird window.

Creating Events:

1. Now whenever you need to create an event (schedule a meeting or create a to-do etc), just click on "New Event" and add details
2. In case you're scheduling a meeting, there is an "Invite Attendees" tab where you can enter and send the emails. It also shows their availabilities and responses.

That's it. It's a handy tool to help you manage your time efficiently and keeping track of your meeting schedules.