Your application to register to vote in Colorado must be received by the close of business today if you want to vote.
FWIW, I am a long time proponent of election day voter registration. Once upon a time, voter registration was justified by the long amount of time it took to get voter rolls together in advance of an election. The logistical justifications for voter registration limitations have long ceased to be relevant. We have a statewide voter registration database on a computer. Now, voter registration is just one more excuse to deny people their right to vote.
I am also a strong advocate of pro-active efforts by government to register people to vote. Motor-voter laws were a beginning, but there is no reason why there shouldn't be a small team of public employees in every jurisdiction using change of address forms and walking neighborhoods all year round to register people to vote. This could be done unilaterally by local governments, giving them an edge in elections generally, at a very modest cost.
Voting has already begun in Colorado, with mail ballots going out starting last Friday, and will continue for more than a month, a phenomena that is among the most important in reshaping Colorado politics. Something like half of Colorado voters have already voted on election day, so the nature of get out of vote campaigns has changed and the importance of late breaking news to election results has declined.