The deadline, and why it exists
Within a day; two at the outside. The note is not an assessment of the evening, it is the receipt of a kindness, and a receipt that arrives three weeks later has stopped doing its work. A host who is still washing glasses on the Sunday is the person the note is written for.
Where a guest travels the next morning, the note is written before leaving, left on a hall table, or posted from the road the same day. Where the household corresponds by email, the email goes the next morning and is short. A message sent on the way home at midnight is warm but is not a letter, and in a house that entertains formally it will be noticed as the difference.
Name one thing
A thank-you that says only that it was lovely gives a host nothing to keep. Name the thing: the mutton, the table, the fact that you were seated next to someone you had wanted to meet for two years, the flowers she had put in the hall because she remembered a remark made in March.
One thing, described plainly, is worth more than a list. A letter that praises everything reads as a letter written without attention, and attention is the entire currency of the form.
Never ask for anything
No request for a recipe, no question about the other guests, no invitation issued in return as a way of settling the debt. The note is not a transaction and does not need to be balanced. An invitation issued in the same letter turns a thank-you into a negotiation.
Likewise no criticism, however small and however gently put, and no remark about a guest who will not read the letter. A host reads a thank-you on behalf of an evening and hears any qualification as the whole of it.
The line that is never used again
A house keeps one thank-you line for the evening itself, said at the door as you leave: thank you both, it was a beautiful evening, and I have never been seated better. Written afterwards, the same words belong in a shorter sentence and with a fact attached.
Flowers sent the next morning are sent by a florist who knows the house, with a card in the sender’s own words and no vase. Nothing that must be dealt with on the morning after is a kindness.
Where the note is not expected
Among close family the note is replaced by the telephone call the next day, and among friends who dine together every month by a line at the door. Where the household is formal, however intimate the friendship, the note is still written: formality and affection are not opposites, and the note is what the house offers instead of a longer visit.