Context: The preferred name for the holiday gradually changed from "Decoration Day" to "Memorial Day," which was first used in 1882. Memorial Day did not become the more common name until after World War II, and was not declared the official name by Federal law until 1967. On June 28, 1968, Congress passed the Uniform Monday Holiday Act, which moved four holidays, including Memorial Day, from their traditional dates to a specified Monday in order to create a convenient three-day weekend. The change moved Memorial Day from its traditional May 30 date to the last Monday in May. The law took effect at the federal level in 1971. After some initial confusion and unwillingness to comply, all 50 states adopted Congress' change of date within a few years.

Question: when was decoration day changed to memorial day?
Steam of consciousness below:

To answer this question, we should know that: Memorial Day did not become the more common name until after World War II, and was not declared the official name by Federal law until 1967.
So, the answer is 1967.