I am not sure if this was something special for the premiere of "A Hard Day's Night" in London on July 6 1964, or if it was something that happened in all of the major cities. But the girls that were hired (or volunteered) to be vendors at the movie and sell programs and souvenirs all wore these Beatles dresses.
For the London premiere, 12 girls in their late teens/early 20's were chosen to be the vendors where they wore the dresses that were either blue or pink. There appears to be 3 different styles. After the movie, they all were allowed to go back to the Dorchester and attend the party with the Beatles. Many of the girls got their dresses signed by the 4 Beatles (see photo). These dresses were sold at department stores and through catalogs later in 1964.