From the U.S. Census Bureau:

JULY 22, 2020 — The U.S. Census Bureau is sending reminder postcards this week to an estimated 34.3 million households. This will be the final mailing before census takers begin visiting nonresponding households across the nation in mid-August. Responding now minimizes the need for census takers to visit homes to collect responses in person.
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According to the Census Bureau’s online map that tracks the nation’s participation in the census, more than 92 million households (or 62.3 percent of households) have already responded online, by phone or by mail.

The COVID-19 pandemic delayed the nationwide start of census taker visits from mid-May to mid-August, giving the Census Bureau the chance to send one more reminder to households encouraging them to respond on their own. A select number of areas will be part of the soft launch of the census taker visits as early as mid- and late July. The postcard is one reminder in a series of reminders that the Census Bureau has mailed nonresponding households since mid-March urging them to respond.

The Census Bureau strongly encourages the public to respond online at 2020census.gov. Households can respond online or by phone in English or 12 other languages. Households can also respond by mail using the paper questionnaire that was mailed in April to most nonresponding addresses. Households can continue to respond on their own until these visits conclude on October 31.

Households that have already responded may disregard the reminder postcard if they receive it. A small number of households that have already responded will receive a census taker visit as part of 2020 Census quality checks.