I actually wrote back to them to ask about the confirmation letter, because we were both confused by the instructions.
Date selected/confirmed in November, they said the confirmation letter would be "January or February," letter arrived in mid-February.

Sorry! But it was a date that was quite a long ways off, so maybe they prioritize those which are coming up soonest? They definitely *did* reserve the date, although they say it's not "official" until the first deposit is sent in (after that letter arrives).