I don't think there ever was a 70's Theme Day, it just said 70's Party and it was listed under Theme Nights.
So a 70's Party at night says Disco to me!
What we originally got from Olivia was:
Theme day: Join us for our theme day – we’re calling it “Show Us Your Roots”! Wear a t-shirt or hat that represents your home town. This will be a great way to meet new friends from your region.
Theme nights: Take part in our exciting theme nights!
Formal Night – dress to impress
White Party – don your whitest whites and let’s light up the Caribbean sky
70’s Party – let’s take it back to the decade when Olivia was born
And the email we just got says:
Show Your Roots Day and Disco Fever Night!
It's all good!