The Error-Proof Portfolio: Dollar-Cost Average or Invest a Lump Sum in an IRA?
The right answer depends on your tolerance for short-term volatility as well as what you'll be investing in, among other factors.
It's IRA season. Of course, technically you can invest in an IRA any time you want--at the beginning of a given tax year, when you get your bonus, or on April 15 just as you turn in your tax return for the year prior. (You have until your tax-filing deadline to make an IRA contribution that will count toward the previous year.)
In reality, however, many IRA investors rush their contributions in right around tax time, so that they can deduct their contribution on their tax return for the year just past or, if they're not eligible for a deduction, so that the contribution can count toward their maximum allowable contribution for that prior year.