The AP has put together an interactive for each state. Its most notable feature is a countdown to various deadlines in each state, which saves you from doing math.
A guide built with college students in mind, the guides found here nevertheless have vital information for how to register and vote. This site is particularly good with information on what acceptable forms of ID are in each state, and how registering to vote can affect other things, such as your status with the Department of Motor Vehicles.
From the statisticians that cover everything from politics to sports, this guide contains important deadlines and links for how to register, vote early, and vote absentee in each state. What makes FiveThirtyEight's guide stand out is that it's updating its information based on recent legislation, executive actions, and court decisions around voting in 2020, so you know where each state currently stands.
A partisan, Democratic-leaning site, but it has all the necessary voter information, including how and where to return absentee ballots without using the mail, something other sites lack.