For an official wedding in Venice, you need to get a certificate from the Russian consulate about the absence of obstacles to marriage in Italy (Nulla Osta). You need to send a package of documents to the consulate no later than one and a half months before the wedding.
Also, for an official ceremony in Venice, you will have to study local laws in advance. For example, according to Italian law, a woman can remarry at least 300 days after a divorce. An excellent alternative to the official ceremony is a symbolic wedding in Venice, which does not require additional documents.