How do I set custom reminders to only occur on weekdays?

Unfortunately, you cannot do that with a single reminder on an iOS device. You would need to set up a reminder for each day and repeat them weekly, if you have any suggestions that you think might enhance iCloud you can send Apple your feedback here.

Currently, the only way to do this is through Siri and you must have an iPhone 4S or higher.

In the Reminders App...

Select Repeat

Select Weekly

Select Custom

Select the days, e.g. Monday, Tuesday, Wednesday, Thursday, Friday

You do realise you are answering posts that are 3 ½ years old.

Why not? I'm reading a post that's 3 yrs old. I found the answer as I often do on old posts. I don't understand the occasional comments like this. Why do you care?