Numerous First Day Covers exist, which I have tried to split up in two sections. I will not detail the differences except if any.

Official Expo 58 FDC cachets

The following 2 covers are quite the same, but differences are noticable (text under and above cachet):

Generic FDC cachet (used for other Italian stamps as well):

Specific mentioning of the Italian Days of 19 and 20 July.