I don't really know but these two sites have some basic ideas about it. I don't think anyone really knows !
Why Can't You Wear White After Labor Day? | Mental Floss
mentalfloss.com/.../why-cant-you-wear-white-after-labor-da.
Behind the No-White-After-Labor-Day Rule - Village Voicewww.villagevoice.com/.../the-reason-s-behind-the-no