If your employer's tax year ends Dec. 31 then Dec. 1 is the date by which you must be enrolled in your employer's Health Savings Account (HSA) Plan in order to be eligible for 2018.
From the IRS...
"Under the last-month rule, if you are an eligible individual on the first day of the last month of your tax year (December 1 for most taxpayers), you are considered an eligible individual for the entire year. You are treated as having the same HDHP coverage for the entire year as you had on the first day of the last month."
Eligibility is defined as having coverage through a high-deductible health plan (HDHP). The maximum contribution amount is $6,750/year per-family and $3,350/year per-individual. If you are age 55 or older you may contribute an additional $1,000.
Beware! Certain clawback rules will both tax and assess a 10% penalty on your otherwise tax-free HSA contributions should you join a HDHP mid-year and then leave it midway through the following year. These circumstances are nuanced -- speak with your financial or tax advisor for more information.
Why HSA? The primary benefits of a HSA plan are flexibility and tax-efficiency. You take control of your health care costs by funding an account with money that can either be used toward qualified medical expenses or rolled over into future years should you not need the funds that year. Your contributions made into the plan (detailed above) are tax-exempt, provided you eventually do use them toward health care. Once you reach age 65 you may spend the funds penalty-free (but not tax-free) in any manner, or you may continue using them tax-free for future medical expenses.