The first mention of Ladies' Day comes in Chapter Three. From the way Greenwood describes it, Ladies' Day is "the big women's magazine that features lush double-page spreads of Technicolor meals..." (27-28). Then, on page 30, Greenwood says, "None of our magazine editors or the Ladies' Day staff members..." which further illustrates the point. Lastly, when recovering from ptomaine poisoning, "Those dodos on Ladies Day" sent the interns get-well cards and copies of The Thirty Best Short Stories of the Year (52). This might be generally known, but in my itinerant interneting - and in some published sources - I have seen it stated that Esther works for Ladies' Day, and it just is not so.
Plath herself confirmed that Ladies’ Day was not where the heroine of her story worked. In a letter concerning libel issues that Plath sent to her editor (James Michie) at Heinemann on 14 November 1961--which is now held as part of the Sylvia Plath Collection at Smith College--Plath states this quite clearly. Unfortunately it is not possible to quote from the letter, but if you are interested in reading it, the good people in the Mortimer Rare Book Room can make a photocopy for you.
And so clearly, Plath fooled around with the actual elements of this incident in her novel, transforming (masking) what took place at BBDO by setting the scene at the offices of a magazine.
Did you know that the June 1953 ptomaine poisoning was not Plath's first experience with the ailment? For her novel, there is no evidence Plath was drawing on an additional, previous experience in the ptomaine scene; however, on July 3, 1946, while she was at summer camp, a quarter of the camp, including five people in Plath's Cove unit, became ill after eating bad fish. The counselors had to spent their time tending to all the children who were fed soup to settle their stomachs. Read more about Plath's time at Camp Helen Storrow in 1945-1946 in my paper "A Perfectly Beautiful Time: Sylvia Plath at Camp Helen Storrow."
All page number references to The Bell Jar in this post come from the 1971, Harper & Row edition.