How do you pronounce Dies Natalis Solis Invicti?

For classical Latin, basically it is pronounced exactly like it would be in English, except the vowels are different.

A is pronounced a long A as in "Ah ha!"

E is pronounced as a short A, as in "Ale"

I is pronounced as a long E as in "Easy"

O is pronounced as a long O as in "Oh no"

U is pronounced EW, as in "Jew"