Use Rainlendar together with Caldavserver Baikal. Here are configured various calendars.
Rainlendar offers a local backup of each calendar.
Is it possible to program this backup, similar to the backup of the settings of Baikal, so that each calendar can be backed up e.g. 28 days backwards, instead of having only one backup?
Rainlendar is not very good app for backing up external calendars since it only stores the information that it needs so restoring such file will not get all your data back.
But if you still want to do it you could create a script which takes a copy of the offline file every day or use a backup tool that preserves the old backups for given amount of days.
Rainlendar’s offline files can be found from here: http://www.rainlendar.net/cms/index.php?option=com_easyfaq&Itemid=26#faq12