Something like this may work:
ROUND((((TODAY() - [Event Date]))/28)-0.5,0)
So you are returning the nearest whole number (of days divided by 28 )
It's a bit of a fudge as Tableau doesn't have a TRUNC function
If you are retrieving data from a database then you may be better to use DB functions to do this.
in addition if the number of days I have doesn't divide by 28 I want to see the rest in the start, meaning in the earliest dates.
if I'm still don't clear pls let me know.. it's alittle complicate to explain