The December 7 gala premiere, the most anticipated cultural event in Milan, would have featured Don Carlo this year.
The strike has been called by Fials, the trade union representing most of the theatre employees, which objects to an agreement signed by La Scala management and the opera house's three other unions in July.
"We are continuing the strike," a Fials official said as quoted by the Italian news agency late on Friday.
La Scala has cancelled a number of performances because of strikes in recent months. In late November, the theater abandoned the three performances of Franz Lehar's The Merry Widow because of a strike by the same union.
It's the second consecutive year that labor disputes have plagued the famous theater casting a shadow on its seasons.