Kbase Article of the Week: Applications With Calendars Cannot Add March Events With Abbreviated Month Name →

Apple Support:

You may experience difficulty when adding events for the month of March in applications with a calendar feature, such as Microsoft Entourage v. X. This occurs when the date region is set for Belgium, Italy, Netherlands, or Switzerland (Italian).

Note: This issue occurs any time you schedule an event for March, whether or not it is March when you attempt to schedule the event.