So you want to set up the data-date and event-date in a table separate of the big dataset. As long as you have a field to blend on you shouldn't have any problem creating some sort of calculation that pops up the correct tooltip. It's probably best if it's a date field. Then you could create something like:
IF [Data_Date] <= [Event_Date] AND [Data_Date] >= DATEADD('day', -7, [Event_Date]) THEN [Event_Description] ELSE "" END
You get the idea. Does this help? Or did I misunderstand your question?