Open enrollment is a window during which individuals and employees may add or drop their health insurance, or make changes to their coverage. Under the ACA, individual health insurance has open enrollment windows, which apply both in and out of the exchanges (prior to 2014, people could apply at any time, but had to go through medical underwriting).
General open enrollment in the individual market for 2018 coverage runs from November 1, 2017 through December 15, 2017, with all plans effective January 1, 2018. This is a shorter window than originally scheduled, and the new time frame was laid out in the market stabilization rule that HHS finalized in April 2017. State-run exchanges that use their own enrollment platform have some flexibility in terms of open enrollment dates for 2018 coverage; Colorado announced that their open enrollment will be longer than other states for 2018 coverage.
Individual market open enrollment in every state (including states like Colorado that extend the open enrollment window for 2018 coverage) is scheduled to use the November 1 to December 15 window for 2019 coverage and beyond. Even if the ACA is eventually changed via legislation, the new structure would continue to need an open enrollment window, as it’s politically untenable to return to the days when eligibility for individual market coverage was based on medical history in nearly every state (under that sort of system, there’s no need for open enrollment, as sick people are unable to obtain coverage in the individual market). The November 1 – December 15 open enrollment window (or something similar) is thus likely to remain in place, regardless of changes made to the ACA.