By default CalendarMenu tries to read all birthdays from your Address Book (Contacts.app). If this is not possible - for example due to missing access permissions (see below), it tries to use the system birthday calendar instead. This calendar however has to be enabled in iCal (Calendars.app) first before it becomes available to other applications; it is disabled by default. This can be done in the Calendar.app preferences window (at the very bottom on the tab "General")
Due to technical limitations there is (at least for now) no way a for a today widget to directly access calendars and reminders. The framework called "EventKit", which is required to do this, is simply not available here.
So to update its data, the today widget of CalendarMenu has to fetch it from the main application; and for this reason the main application of course needs to be running.
It goes without saying that CalendarMenu will be updated if this limitation should be removed in the future.
That's the reason why the today widget shows a button to directly launch the main app from within the notification center, if it should not be launched at the moment.
Please note that the today widget is only available in OS X version 10.10 (Yosemite) and newer.
In OS X version 10.9 (Mavericks) and newer, the calendar app automatically subscribes to a holiday calendar matching the current region and language settings of your system. But they only show up in the calendar app (which is required to show them in CalendarMenu) if they are enabled in the application preferences of the calendar app.
Please note: these calendars apparently are not available for all regions and countries.
If you want to use these calendars on previous versions of OS X like Mountain Lion, you can either use iCal subscr"+"iptions from different sources (which also may provide better data) or you can use the Apple feeds as well. The URI of these feeds can be used for manually adding them as calendar subscr"+"iptions and is as follows:
You just have to replace the ISO region code (in this case "DE_de" for Germany) with the desired one, for example "CH_de" for Switzerland or "US_en" for the United States.
If you use iCloud for your calendars and reminders, this data will be synchronized automatically by OS X. Changes will be shown in CalendarMenu as soon as OS X synchronizes itself with iCloud (which may take a few seconds, and of course requires internet connectivity).
So there is no need to activate this option unless there are other calendars which may need to be updated regularly.
If you mark a reminder as (un)finished within CalendarMenu, this change will also be synchronized automatically.
All iCloud calendars and reminder lists will be marked as subscr"+"iption in CalendarMenu.
Due to the App sandboxing model used in the Mac App Store, sandboxed apps have to register Login Items using a system framework called "ServiceManagement". Such a login item is not visible in the System Preferences Panel "Users & Groups", so you can only remove it using the respective application which created it in the first place.
So this is an intentional (but very unfortunate) behaviour of OS X.
Alternatively, you can manually add CalendarMenu to the Login Items in the System Preferences Panel "Users & Groups" instead of using the respective checkbox in the CalendarMenu settings.
On screens with lower resolutions (for example on a MacBook Air) it can happen that applications like Safari, which has many menus, cause a few status bar items to suddenly disappear.
This is automatically done by OS X because it always shows all menus and leaves the available space to the status item area on the right side of the bar. But this space may not be sufficient to show all active status items, so the system starts to hide them until there is enough space to show the others; it usually begins with those on the very left.
Unfortunately there is no way to influence this behaviour, nor can a developer choose where a status item should be shown. It is all decided and done by OS X.
The only possible way to create more space is to deactivate unused status items. Some items - like for example the user menu - also offer reduced appearances which require less space and thus leave more of it to the others.
But CalendarMenu is always accessible with its own global keyboard shortcut (which first has to be set of course), even though it may not be visible at the moment.