You can contribute to your IRA account anytime during the current year and as late as the due date of your individual income tax return, not including extensions. You can make the contribution all at once or dollar-cost–average your contributions, i.e., systematically invest a fixed dollar amount regularly.
SUGGESTION: If possible, make your IRA contribution as early as you can in the year. Missing out on 15½ months' earnings compounded over the next 20 or 30 years can make a big difference. If you can't make the full contribution on January 1, consider dollar-cost averaging.