just because they cant choose the date doesn't mean they're not confident in their decision to ctb.
for choosing the date, honestly it's completly up to you, march is a long time, so you might change your mind. or you may not. i recommend getting closer before deciding. just say your doing it in march for now, and as you get closer, see how you feel. if going through your anaversery seems like it'd be too painful, if you wish to you can do it beforehand. if you feel not necessarily perfect, but like you can get through the anaversary, but you still want to ctb, you can do it on your birthday. or you can choose litterally any other day of the month, doesn't even have to be march. as long as you feel comftorable and happy with whatever you choose.
i've always found putting exact date restrictions on ctbing can be a bit overwhelming for me. especially when i end up failing, i feel like i disappointed the world because i was supposed to be gone that day. just make sure that whatever date you choose, you know your not bound to it by any means, we'll support you no matter what happens.
good luck <3