Title 13 U.S.Code Section 141. Population and Other Census Information.
(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, which date shall be known as the “decennial census date”, in such form and content as he may determine, including the use of sampling procedures and special surveys. In connection with any such census, the Secretary is authorized to obtain such other census information as necessary.
(d) Without regard to subsections (a), (b), and © of this section, the Secretary, in the year 1985 and every 10 years thereafter, shall conduct a mid-decade census of population in such form and content as he may determine, including the use of sampling procedures and special surveys, taking into account the extent to which information to be obtained from such census will serve in lieu of information collected annually or less frequently in surveys or other statistical studies. The census shall be taken as of the first day of April of each such year, which date shall be known as the “mid-decade census date”.
The decadal language is enshrined in Section 13 of the U.S. Code. Paragraph (d) makes it clear that the Congress embraces the idea of mid-decadal assessment and mandates it there. If memory serves, there were discussions and debate about going to a five-year period on the full Census when Section 13 was last revised. If I were being paid to do it, I would dig into the Congressional Record and track down those debates.
Congress enacted Section 13 of the U.S. Code, and Congress can amend it. If they want to amend it to mandate a 2022 Census and a 2030 Census, they can do that.