Specs are quantitative aspects of the option such as the price
Pros
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.
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.