I don't believe there is any real way to do what you are suggesting with the calendar in the DBM.

Years ago I asked for a similar function, user-definable popup messages.

They gave us the Additional Info fields, in each set of ledgers, in response to that (it's not what I was looking for but it has it's advantages).  You can go to the employee(s) in question, use one of the Additional Info fields to put in a message and click the Pop up check box.  Then when you go to select that employee on a paycheque the message will pop up (sometimes behind the screen you are on but you can usually see it in the task bar at the bottom - unless you group your icons).

I think this is the closest you are going to get, unless someone else has any ideas.

It's possible to use the Employee list report and filter based on the dates but you would be changing the filter all the time because you cannot set a dynamic option (eg. Hire date is between 5 and 6 years old from today's date) and you have to remember to run it.

Why not just use Outlook Calender ? Or what ever e-mail program they are using

I considered the pop up using the additional fields but then I have to see it every payroll until December.  I dislike constant pop ups.   I don't use Outlook and my email program doesn't have a calendar feature to handle this.   I suppose I'll have to do it the old fashioned way and write something on my paper calendar hanging on the wall...I just have to remember to look at it! :P