The room number, sent on arrival day
The room number is the last detail of a booking made from home, sent after check-in with anything the building needs; the hotel name is enough until then.
A booking made weeks ahead is made to a hotel, not to a room. The room is decided by the front desk on the day, and so the room number is the one part of the booking that cannot be settled from home. It is sent into the thread after check-in, and it completes the booking.
What the message holds
The number itself, and anything about the building she needs to know. A lift that needs a key card, which means meeting her in the lobby. A side door that takes over from the main one late in the evening. A different wing reached through a separate door. For an apartment, the street and house number, the floor and the name on the bell, since buildings in the old centre often share one entrance between several homes.
When it changes
An upgrade, a move after a noisy first night, a transfer to a sister hotel: each changes the number, and each needs a new message. If the thread holds an old number on the evening, she knocks on the wrong door. Whenever the room changes, the thread should hear about it, even if the booking is days away.
When it comes late
A delayed flight or a slow check-in can push the message into the evening. The booked hour does not move because the front desk was slow, so send the number as soon as you have the key. If check-in is so late that the booked hour cannot be kept, write before she sets out; the cancellation scale and the option of a later start on the same evening both depend on that timing.
What the hotel is told
Nothing. The house does not contact the hotel. The page on booking before you have a room number covers apartments, early arrivals and the other awkward cases, and the journal note on changed rooms covers what happens when the hotel changes its mind.