B'Day remainder before one day (some timing)

Use a timer control in the windows forms that runs every time interval. Whenever timer time is elapsed, handle the event Timer_elapsed code.

In that code connect to your database and check if there is any birthday coming and pop up every time until you close it.
Create a table with column for storing the flag value. If flag is 0 then the window is not closed for the birthday and show popup every time.
When you close the pop up, on the close event of the window update the table flag to be 1.

Let me know if you have any other questions.