You could use a calculated field:
DATEPART('week', [Event Time] )
I've never heard of a week function and can't find it in the available Functions
Ahh now I feel a bit stupid. I should have thought of that...
I don't know what it is called in Tableau, so I called it a "function". It is an option on the datefield. (week, year, day, month++)
I think the Custom Date option on date fields does it slightly differently.