There's a simple way to figure it out even if all you have is a calendar. (And no, I don't mean counting back 180 steps.)
The first thing to know is the right day of the week - If you arrive on a Monday, your reservation day will be a Wednesday. Arrive on Tuesday, reserve on Thursday. Arrive on Friday, reserve on Sunday . . . and so on. Reservation day (not date) is two days (of the week) after arrival day. Suppose in your case, it's a Saturday.
So once you know the right day of the week, then take your arrival date, go back six months, then go forward two days. From there, find the closest Saturday. That will be your reservation date.
say you're arriving on 4/15/12. That's a Sunday. So your reservation day will be a Tuesday. (Two days after Sunday.) From 4/15, back up six months to 10/15/11. Then go forward two days to 10/17/11. That's a Monday. The closest Tuesday is 10/18. That's your reservation date.