The next US census is scheduled for 2020-04-01. In light of the recent COVID-19 pandemic it might become necessary to delay this. How can this legally be done (since the census is required by the constitution)?
The date of the census is not specified in the US constitution. It only says
The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct.
There is no requirement that "ten years" be interpreted precisely to the day. The date is fixed by statute at 13 USC 141(a):
The Secretary shall, in the year 1980 and every 10 years thereafter, take a decennial census of population as of the first day of April of such year....
This requires that the census seek data describing the population on April 1, 2020. It does not require that census workers actually visit people on that day. They could conceivably do it in mid August, for example, or at any other time. Of course, that would probably reduce the reliability of the census. Congress could pass a law moving the census date to another later date if they wanted to get a more accurate count of the population on that date instead of a less accurate count of the population on the first of April.
In fact, when I got home later in the day after writing this answer, there was a letter from the census bureau inviting me to submit my census survey through the internet. I did that a few minutes ago, so if I die in the next two weeks, the census will be off by one.
The letter I got says that if I don't fill out the survey online, they'll send me a paper questionnaire. If I don't fill that out, someone will come to visit. The prospect of that happening on April 1st seems quite remote.