Woman's Day? Every day is a woman's day for me.

If you mean that you will be having your menstrual period on that day, you can use one tampon and expect to perhaps bleed into your tri shorts. Most of them are padded and black, so it won't show. Some women who run marathons prefer to use two tampons (side by side, not on top of each other). You can also ask the RD if there is a changing room/restroom for use during T2. This is uncommon, but it's worth an email.