More than 80,000 non-essential employees are scheduled to return to their workplaces on Monday.
However, the city's largest public union wants that delayed until January.
There's a weekly testing mandate for the unvaccinated that will come with a message that they should get the shots.
DC 37, the city's largest public union, is calling on the city to develop a hybrid schedule.
"The only to get people vaccinated at this point is to use both incentives and strong mandates. And saying we're going to delay, delay, delay, is a formula for telling people they don't need to be vaccinated and that we're not going to fight back and serve people the way we need to. I'm not doing that," de Blasio told WNYC.
The state has pushed back the return of its' employees to October 12.
MORE NEWS: NYC schools set to reopen Monday for in-person learning
MORE CORONAVIRUS COVID-19 COVERAGE
New York City COVID-19 Vaccine Tracker
New Jersey COVID-19 Vaccine Tracker
Centers for Disease Control and Prevention on coronavirus
Submit a News Tip or Question