is the valid Event data field string related to the day of month and that is what allows you to use it in other months? if so, why not just use day of month and skip this field all together? It sounds like you could just create a master table with all days of months and event names and then key off of that as a join across all of your data. The data source would need to support the join but a blend would work too.
Any sample data might help clear up the issue and better allow the community to assist.