You can contribute money to your IRA for one tax year as late as your tax filing deadline the following year. That is, if you put money into your IRA before taxes are due -- typically April 15 for most taxpayers -- you can count it toward the previous year's IRA limits. Beyond that, when you contribute during the year is up to you, as long as you meet the IRS requirements for contributing to your account.
You can only contribute in a year when you earn "taxable compensation." That includes wages, salaries and self-employment earnings but not rental, dividend or interest income. It doesn't include tax-free income except for non-taxable combat pay. Another restriction is that you can't put money in after you turn 70 1/2, the age when mandatory minimum withdrawals start. If you have a workplace retirement plan and a high adjusted gross income, you may not qualify to make tax-free contributions.
If you didn't make any taxable income this year but your spouse did, she can contribute to your IRA for you. After she maxes out her account -- $5,500 is the limit on contributions as of 2013, or $6,500 if you're 50 or older -- she can put as much as she can afford in your IRA, up to the same contribution limit. This option is only available if you file a joint return.
Your IRA contributions start earning money as soon as they're in the account. If you're looking to maximize your retirement income -- and who isn't? -- making contributions early in the year is a smart move. Deposit $5,000 January 1, for example, and your contribution works for you throughout the year. You can legally deposit the money at the end of December, but that denies you 12 months of potential earnings. Wait until mid-April of the following year, and you've missed out on another three and a half months.
One drawback to contributing early is that you may end up putting in more than your allowed maximum for the year. For example, you can't contribute more than your earned income -- salary and wages, as opposed than "unearned" interest and dividends -- so if your earnings are lower than you expected, your contributions may be too much. You have to complete the paperwork to take contributions back out or you pay a 6 percent tax on the excess. Later in the year, you probably know your finances better.