Previous N week data

The simplest way to do this would be just add a calculated column to your table with this formula

RegistrationWeek = DATEDIFF(Table[EventStartDate], Table[RegistrationDate], WEEK)

That will give you # of weeks before the event (-1,-42, etc.).  You can then make a matrix with Event on the rows and the new column on the Columns, and use a running total measure like this

Cumulative Registrations =
VAR __thisweek =
SELECTEDVALUE ( Table[RegistrationWeek] )
RETURN
CALCULATE (
COUNTROWS ( Table ),
ALL ( Table[RegistrationWeek] ),
Table[RegistrationWeek] <= __thisweek
)

If this works for you, please mark it as the solution.  Kudos are appreciated too.  Please let me know if not.

Regards,

Pat