Checking for a week number in a workflow expression

[joebordes]

If you get the SQL I understand that you are creating a scheduled workflow. Scheduled workflows do not support expressions because they would need to be translated to SQL which is VERY complex. In scheduled workflows only basic conditional operations that can be directly translated to SQL are supported.

I would suggest you create a numeric custom field and fill it in with the value in the picklist with an update field workflow and then create the scheduled workflow based on the numeric field.