Colo. Rev. Stat. § 39-11-101
The treasurer shall make a list of all lands and town lots the tax liens on which are subject to sale, describing such land and town lots as the same are described on the tax roll. Except as otherwise provided in section 39-2-117(1)(a), no later than September 1 of each year, the treasurer shall send a notice by mail, at the person's last-known address, to each person by whom taxes for the previous year are known to be due and unpaid. The notice shall indicate the amount of the person's delinquency and state that if the amount of the delinquency is not paid by the date specified in the notice, which shall not be less than fifteen days from the date of mailing of the notice, the treasurer will advertise and sell a tax lien on the person's property on the date specified in the notice at public auction for the delinquent taxes, interest, and applicable fees. If such list is not made until after September 1, the sale held thereunder shall not be void by reason thereof.
C.R.S. § 39-11-101
For time when tax lien attaches, see § 39-1-107.