if statement returning same statement for all results rather than changing based on the if statement

I'm trying to write a simple code that will tell me if the date that I need to do something falls on a weekend. When I run this code I get the "change day" regardless of the day of the week, there is obviously something wrong with the code but I can't see it

if week_Days[movePlates.weekday()] == "Saturday" or "Sunday":
    print("change day " + str(week_Days[movePlates.weekday()]))
else: 
     print("move plates on: " + str(movePlates))


if week_Days[readPlates.weekday()] == "Saturday" or "Sunday":
    print("change day " + str(week_Days[readPlates.weekday()]))
else: 
    print("read plates on: " + str(readPlates))

Change day Sunday
Change day Thursday

This is a result of implicit booleans. To add parentheses to your if statement (without changing it):

if (week_Days[movePlates.weekday()] == "Saturday") or "Sunday":
    print("change day " + str(week_Days[movePlates.weekday()]))
else: 
     print("move plates on: " + str(movePlates))

The problem is that you're treating the string "Sunday" in an or statement, where any nonempty string is True, so the or statement evaluates to True. Here's one solution:

if week_Days[movePlates.weekday()] == "Saturday" or week_Days[movePlates.weekday()] == "Sunday":
    print("change day " + str(week_Days[movePlates.weekday()]))
else: 
     print("move plates on: " + str(movePlates))

Or if you wanted to make it a little simpler with different syntax, you could do

if week_Days[movePlates.weekday()] in ("Saturday", "Sunday"):
    print("change day " + str(week_Days[movePlates.weekday()]))
else: 
     print("move plates on: " + str(movePlates))

You need to test the variable against both days in your if-clause (or use the in() operator). I'd suggest wrapping the test in a function (and using lower() if the casing is not fixed):

def IsWeekend(a):
    return (a.lower() == "saturday" or a.lower() == "sunday")