The magazine Ladies' Day is probably a fictionalisation of the American magazine Woman's Day which was first published in 1931, and is still published today. During the period in which The Bell Jar was set, it specialised in food and cookery articles, and other domestic issues.
Woman's Day was one of a group of seven American magazines, known collectively as the 'Seven Sisters', which were aimed at married women with children. Six of the 'Seven Sisters' group are still in existence.
Sylvia Plath's choice of name may also be influenced by the Ladies' Home Journal, the title of another magazine in the group.