The rally will see leaders of 12 far-right parties marching towards their hoped-for conquest of Brussels after next week's European parliamentary elections.
Headliners Salvini of the anti-immigrant League and Marine Le Pen of France's Islamophobic National Rally (RN) hope their Europe of Nations and Freedom (ENF) group will become the third largest in Brussels.
But despite their shared dislike of immigration, multiculturalism, the left and the EU, Europe's populists remain divided on many other key issues, including budgetary discipline, migrant distribution and relations with Moscow.
Most of Europe's rightwing nationalists are currently divided into three blocs and a tangled web of alliances in the European Parliament, which Salvini and Le Pen would like to overhaul if not destroy.
The ENF includes Austria's Freedom Party, Belgium's Vlaams Belang and the Netherlands' Party for Freedom, whose head Geert Wilders will be in Milan.
Thousands of League supporters will march towards Milan's Duomo (cathedral) square where leaders will make their speeches from 1430 GMT.
Notably absent from the rally is Hungary Prime Minister Viktor Orban's Fidesz party and Poland's governing PiS (Law and Justice party).
Orban has voiced admiration for Salvini and promised “cooperation” after the vote, but refuses any alliance with Le Pen.
Smaller parties such as Bulgaria's Volya or Slovakia's Sme Rodina, which is set to win a single MEP seat, are to join the Milan rally, held alongside a large anti-fascist demonstration.
'Life or death'
“Give us a hand to become the top European party, to take back the keys to our home,” Salvini, who has been frantically campaigning around the country, told a recent rally.
“The European elections are a referendum between life and death, between the past and the future, a free Europe and an Islamic state based on fear,” he said.
Italy's La Repubblica newspaper said on Friday that Salvini might win seats in the May 23-26 European elections but his dream of uniting nationalist parties was “unrealisable”.
While Salvini and Le Pen have close ties to Russia and President Vladimir Putin, eastern European far-right parties are wary of Moscow's ambitions.
Critics feel that an enduring alliance between the League and Germany's Alternative for Germany (AfD), which will be in Milan, would be impossible.
“Salvini for instance wants a European redistribution of refugees, (lead candidate Joerg) Meuthen doesn't want a single refugee. What's more, Meuthen doesn't want to give a single cent to southern Europe,” leading German Green Party member Sven Giegold told Italy's AGI news agency.
Salvini hopes the future right-wing bloc will be able to implement laxer EU budget rules, a boon for Italy's struggling economy.
“I think that the new figures in the European parliament and the new balance in the Commission will allow the rules that are strangling the economy to be changed,” Salvini said on Friday.
Salvini's coalition relationship with the anti-establishment Five Star Movement of fellow Deputy Prime Minister Luigi Di Maio is increasingly fraught since they formed a government last June.
Opinion polls predict that the League will go from six MEPs to 26, Le Pen's RN from 15 to 20 and AfD from one to 11.