Ranked in these QuestionsQuestion Ranking
Pro Free, open source and cross-platform
Lightning Calendar (just as Thunderbird) is completely free and open-source meaning that anyone can use it as well as look through the code or even use the code for their own projects.
Pro Easily keep track of tasks
Bundled in with the calendar options is a tasks option that makes for an easy way to keep track of tasks in the same location as ones events.
Con Available only as part of Thunderbird
No standalone version of the calendar app is available. And if you're just interested in Lightning Calendar and not Thunderbird, the app will have additional functionality and interface elements that will get in the way of using the calendar.
Con Not to all the versions
I had to downgrade my Thunderbird just because Lightning didn't upgraded to the last beta version....and I can't live without Lightning.