To be eligible for HSA contributions for a month, an individual must be covered under an HDHP as of the first day of the month and have no other impermissible coverage.
For example—HDHP Coverage Begins Mid-month: An employee begins HDHP coverage on the first day of a pay period, which is August 16, 2014, and continues to be covered by the HDHP for the rest of 2014. For purposes of HSA contributions, the employee becomes eligible on September 1, 2014.
An HDHP is a health plan that provides “significant benefits” and satisfies requirements for minimum deductibles and out-of-pocket maximums. An HDHP can be insured or self-funded. With the exception of preventive care benefits, no benefits can be paid by an HDHP until the annual deductible has been satisfied.