That's up to you. December will be easiest, of course, since it will match the availability of the various forms and instructions. But if your father's estate will have income in 2024, you might need to file estate returns for both 2023 and 2024. If you picked May April 2024 (the latest date you could pick), and if your father's estate is ready to close by then, you would only have to file one estate return.
