WEEKDAY vs. WORKDAY explained
WEEKDAY
Imagine you've got a dentist appointment on October 15, 2018. You typically work Mondays, Wednesdays, and Fridays, so you'll need to know if you should request off work for this appointment.

Using a calendar, we'd go back to the given date and determine that it's on a Monday, so we'd need to request off work.

The WEEKDAY function does the exact same thing!
Given a date, the WEEKDAY function returns a number between 1-7 representing what day of the week the date is on.
WORKDAY
The WORKDAY function, by contrast, is completely different. It's easy to get them twisted though, so let's visualize the difference.