The start of in-person learning in New York City public schools has been delayed to Sept. 21, Mayor Bill de Blasio announced Tuesday.

The district's blended-learning plan was scheduled to begin on Sept. 10.

"Teachers who usually get two days of professional development at the beginning of the school year will now get nine," schools Chancellor Richard Carranza said. "We've heard from everyone in our schools that have said we need some more time."

In averting a strike authorization vote by the United Federation of Teachers later Tuesday, de Blasio agreed to delay the start of school to Sept. 16 for all-remote instruction. In-person learning, which is limited to a few days per week, will begin Sept. 21.

The mayor also announced expanded testing for teachers, a significant concession to the United Federation of Teachers.

ABC News' Aaron Katersky contributed to this report.

Alaska and Montana have been added to New York's travel advisory list, which requires people traveling to the Empire State from a high-coronavirus area to quarantine for two weeks, New York Gov. Andrew Cuomo announced Tuesday.

The states and territories currently on New York's list are: Alaska, Alabama, Arkansas, California, Florida, Georgia, Guam, Hawaii, Iowa, Idaho, Illinois, Indiana, Kansas, Kentucky, Louisiana, Minnesota, Missouri, Mississippi, Montana, North Carolina, North Dakota, Nebraska, Nevada, Oklahoma, Puerto Rico, South Carolina, South Dakota, Tennessee, Texas, Utah, Virginia, the U.S. Virgin Islands and Wisconsin.

The list applies to areas with a positive test rate higher than 10 per 100,000 residents over a one-week rolling average or areas with a 10% or higher positivity rate over a one-week rolling average.

New York was the U.S. epicenter of the pandemic in March and April, but of those tested across New York state on Monday, less than 1% were positive, Cuomo said.

New York currently has 109 coronavirus patients in ICUs -- the lowest number since March 15.

White House public tours will resume on Sept. 12, though the number of visitors will be limited to 18% of the usual capacity, according to a Tuesday statement from the first lady's office.

Tours will also be cut down to two days per week instead of five days per week.

Everyone ages 2 and older must wear a face covering and practice social distancing.

The Bureau of Prisons said Monday that it's resuming in-person visits at all 122 of its facilities, according to an internal memo obtained by ABC News.

"Social visiting will resume no later than Saturday, October 3, 2020," the memo said, adding that visiting will be non-contact only.

"Use of Plexiglas or a similar barrier between inmate and visitors will prevent any contact. In the alternative, if a barrier is not present, social distancing (i.e., 6 feet apart) between visitors and inmates must be enforced," the memo said.

Inmates and visitors must wear face coverings at all times.

Every inmate will be permitted two visits per month. Inmates in isolation or quarantine will not be allowed visits.

ABC News' Luke Barr contributed to this report.