what is the meaning of this sentence?

Yes, the word toilet is being used as an old-fashioned word that means clothes, and not bathroom. So, the sentence means, "The women went to the garden party to be seen in their summer clothing."

snicker

what did *you* think it meant, lil prince?

Should be spelled "toilette" because 1) we are referring to their manner of dress; and 2) we are referring to women. (The word comes from French.)

A woman "makes her toilette" (and a man, his "toilet") by bathing, powdering, dressing, putting on makeup, dressing her hair -- getting ready for the day. Obviously, functions of bodily waste elimination are included in there somewhere, but, being polite, we don't mention that... Hence, the use of "toilet" to mean a place to accomplish these acts, and the "unmentionable" equipment therein...