Today was a milestone in New York City’s effort to make free, full-day pre-K available to 3-year-olds: The first round of offer letters went out to parents.

The city is expanding its popular Pre-K for All program to start a year earlier — with 3-year-olds. The effort is dubbed 3-K for All.

“I am convinced that this is one of the most important things that the city can do for our future,” Mayor Bill de Blasio said at a press conference. “The sad reality is that in our school system here and in most of the country, we kind of had it backwards for many, many years. We ignored the early childhood opportunity.”

On Thursday, the city shared some first figures on the program. Here’s a look.

2,321: number of families who applied for a spot

The city is starting its efforts with pilots in two school districts: District 7 in the South Bronx, and District 23, which covers East New York, Brownsville and Ocean Hill. It will take two years to achieve universal access in just those districts, which the city says will require 1,800 seats.

At full capacity, the city hopes to serve 62,000 students citywide by 2021, but achieving universal access will require significant funding from the state and federal government.

793: number of offers sent on Thursday

Though fewer than 800 students received offers Thursday, there are many more 3-year-olds who will attend pre-K. The Administration for Children’s Services already provides childcare for low-income families in the district and throughout the city. Those programs have income restrictions, while 3-K for All is open to any New York City resident with a child born in 2014.

Between both efforts, the city says it will serve 1,600 3-year-olds in the pilot districts this fall.

84 percent: share of families in the two pilot districts who received offers out of those who applied

While 3-K for All is kicking off in only District 7 and District 23, families from anywhere in the city were able to apply. However, families living in the pilot districts were given priority in admissions for some 3-K for All programs.

$16 million: cost of the expansion in the two pilot districts

The city added almost 800 seats for 3-year-olds in the pilot districts.

In addition to the new slots, the education department is also providing teacher training, social workers and other support for existing centers run by ACS. The goal: ensuring quality, as well as continuity for children going on to city pre-K programs for 4-year olds. In all, the city expects to serve 11,000 3-year-olds this fall.

Update: This story has been updated with more recent figures for the number of new pre-K seats that have been funded by the city.