Missing the deadline to take money from your retirement accounts could trigger a nasty 50 percent tax penalty. Emphasis on "could."

Under IRS rules, so-called required minimum distributions (aka RMDs) generally kick in on IRAs once you reach age 70½. For 401(k) plans and other defined contribution plans, it's typically when you turn 70½ or you retire, whichever is later.

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The deadline to take a particular year's RMD is Dec. 31 of that year. But in the year you're first required to take an RMD, you have until April 1 of the following year to make that withdrawal. If you have cash in your account, Fidelity says you can take that withdrawal "any day of the week" — including the deadline itself, Saturday. Need to make a trade? You're likely too late. (For more of the ins and outs on RMDs, including how to figure out how much to take, see our guide here.)

Attach a letter explaining why you missed taking it last year. Usually the IRS has waived the penalty. Maura Cassidy vice president of retirement, Fidelity