If you have FMP v11, you can use a single self-join relationship and portal filters, with no need for extra fields.
In the Relationship Graph, create another occurence of your Events table and call it Events_2.
Then set up a self-join relationship like so:
Events::Day Of Week >----X----< Events_2::Day Of Week
The X is a "cartesian" join, not the default = sign.
Set up each portal to show related records from the Events_2 table.
Then set up a filter on each portal:
Specify first portal filter as Events_2::Day Of Week = "Monday"
Specify second portal filter as Events_2::Day Of Week = "Tuesday"
and so on...