Update: Lightning is now already integrated into Thunderbird
Mozilla Lightning is a free calendar solution for the Thunderbird email client. You can easily add appointments and tasks and you can choose to be reminded by an alarm in case of important events. The calendar offers a daily, weekly and monthly view.
Mozilla also provides holiday calendar files for many countries available for download which contain national holidays. You can either download and import a calendar into Lightning or you can simply subscribe to a calendar by copying the URL and then adding it as a new remote calendar file. More information on installing a holiday calendar can be found in the Adding a holiday calendar article.
We recommend to install the calendar file locally in Lightning. With this method, the calendar file is stored on your hard disk and will be available even if you are offline. You will also be able to modify it (for example, by adding provincial or state holidays to a national holiday calendar).
Available for: Windows, Mac OS X, Linux