Article 3 of Decree 23 luglio 2021, n. 105 prescribes the conditions for visiting a fair: Only persons with a valid "Certificato Verde" (Green Pass) may enter a tradeshow. Here is the link to the original text of the decree (in Italian):
Article 9 of Decree 22 aprile 2021, n. 52 defines what is certified with the "Certificato Verde" ("Green Pass") (Vaccinated, Convalescent, Tested) and for how long the respective certificate is valid (Vaccinated/ Convalescent: 6 months, Tested: 48 hours). Here is the link to the full text of the decree: