Easy --- It's the National Voter Registration Act:

Pertinent Excerpt:

Does Section 8 impose any time restrictions on States as to when a general list maintenance program can be conducted?

Yes. Section 8 requires States to complete any program the purpose of which is to systematically remove the names of ineligible voters from the official list of eligible voters not later than 90 days prior to the date of a primary election or general election for federal office.

The full link:

https://www.justice.gov/crt/national-voter-registration-act-1993-nvra