Generally, RMDs have to be taken on or before December 31 of each year beginning with the year you attain age 70½.
The IRS does provide a grace period for the first year of your RMD, therefore distributions can be taken as late as April 1 of the year following the year you reach 70½. For example, if you attain age 70½ in 2011, you have until April 1, 2012 to take your RMD for 2011. However, your RMD for 2012 must be taken before December 31, 2012. So, if you opt to do this, you will take 2 RMDs within 1 calendar year. All subsequent distributions must be taken before December 31 of that year.