@Mayukh Bhattacharya - Thanks, but despite how this seems logical, it's not working for me.
I have applied the Conditional Formatting formula:
="IF(OR($G$3=""Sat"",$G$3=""Sun"")"
to rows:
=$5:$500
(By the way, I can't seem to respond to answers here on Stack Overflow - despite me having used the site for many years. There's been a recent change, but I don't know why it's affecting my ability to comment) on answers.)