Conditional Absence Management task expiration

Summary

How does one create a conditional expiration time based on the day of the week.

Content

for absence management task in BPM we would like the task to expire after a certain number of work days (e.g. 3). To do this, we need to write a conditional expression, but we are not sure what to put in. I was thinking the statement might look like

IF Week_Day = Wednesday OR Thursday OR Friday THEN Expiration_Duration = 5.

What we don't know are the variables to use in the expression (e.g. Week_Day).