You are considered covered by a retirement plan all year if only covered for one day. You can always contribute to a Traditional IRA if you have earned income even though your deduction could be limited because of the employer's plan.
Please see this IRS link for IRA deduction limits when covered by a retirement plan at work, to determine if your contribution is limited. https://www.irs.gov/Retirement-Plans/IRA-Deduction-Limits
