It's mentioned in Pyramids and Going Postal.
Pyramids wrote: Octeday afternoons was Political Expediency with Lady T'malia, one of the few women to achieve high office in the Guild. In the lands around the Circle Sea it was generally agreed that one way to achieve a long life was not to have a meal with her Ladyship. The jewellery of one hand alone carried enough poison to inhume a small town. She was stunningly beautiful, but with the kind of calculated beauty that is achieved by a team of skilled artists, manicurists, plasterers, corsetiers and dressmakers and three hours' solid work every morning. When she walked there was a faint squeak of whalebone under incredible stress.
Going Postal wrote: ‘I said that's enough, Stanley! Now, just raise that lamp again, will you? Good. That's better. I'll read a page of the Regulations, that always quietens them down.' Groat cleared his throat. ‘I shall now read from the Book of Regulations, Delivery Times (Metropolitan) (Sundays and Octedays excepted),' he announced to the air. ‘As follows: "The hours by which letters should be put into the receiving houses in town for each delivery within the city walls of Ankh-Morpork are as the following: overnight by eight o'clock in the evening, for the first delivery. Morning by eight o'clock, for the second delivery. Morning by ten o'clock, for the third delivery. Morning by twelve o'clock, for the fourth delivery. Afternoon by two o'clock, for the fifth delivery. Afternoon by four o'clock, for the sixth delivery. Afternoon by six o'clock, for the seventh delivery." These are the hours, and I have read them.' Groat hung his head for a moment, and then he closed the book with a snap.
I think that, once he established the system in the first book, kept it but just didn't mention it again.
This way he didn't need to actively change anything (and therefore admit a mistake) or burn any bridges for future ideas.
On a side note, did you know that the calendar that we use now is based on the Roman Calendar?
It had 10 months (March through December) and 300 days and one of the emperors decided that this was stupid and added two new months to the beginning of the year. This year had over 400 days in it and was known as "The year of confusion" because nobody knew what the date was!
This is why you meet people named March, April, May, June, July, and August (like Augustine or Agustus), because those months were named after gods or kings or something.
September through December are just numbers. "September" actually means
"seventh month". This is why you never meet anyone named January, February, September, October, November, or December.