The Crucial Step
Up until the moment that the Japanese government decided to adopt them for the new Western-style week, the planetary names had little more than astrological significance in Japan. They were in no way comparable to the Western days of the week, which had had official status in the calendar since AD 321.
After the guchureki-style calendars (具注暦 guchūreki, see note) went out of use, for much of the second millennium the planetary names in Japan were mainly used in kanagoyomi (仮名暦, kanagoyomi), calendars in the kana script used by women. The kanagoyomi recorded only the planetary name for the first day of each month. It was not until 1873, with the adoption of the solar calendar from the West, that every day of the month came to be marked with the name of the day.
In March 1876, the Western-style week was decreed for public use, starting the following month. This meant that the seven-day week became a part of everyday life rather than just an astrological curiosity. However, it was many years before industry moved to the custom of taking Sunday as the day of rest.
While the adoption of the seven-day week is understandable from the point of view of modernisation (or Westernisation), it's not clear why the old planetary names were adopted to name them. Perhaps it was the contemporary practice of turning to classical written Chinese to render the new concepts that were being imported wholesale from the West. The adoption of the planetary names, taken from Chinese tradition, was very much in tune with the age. (It is likely that the Japanese were acting on a hint from the Chinese themselves. In China, the term 星期 xīngqī meaning 'week' was created by people aware of the old names ending in 曜日 yàorì.)
While it may have been standard practice, however, the adoption of the planetary names was by no means a foregone conclusion. As the Chinese example shows, there were alternatives. Indeed, were this decision to be taken today, more than likely the English names would be adopted. The Japanese week would be very different if the days were named 'Sandē' 'Mandē' 'Chūzudē' 'Wenzudē' 'Sāzudē' 'Furaidē' and 'Satādē'!
At the end of the day, the decision to use old astrological categories to render the new-fangled calendar from the West should be regarded as something of a 'creative leap'. Without this step, the long journey from the West through India, China, and one thousand years of Japanese tradition would have ended as a mere footnote to history. By having taken this step, the Japanese can claim a place alongside the languages of the West as heirs to the ancient day names.
The concept of a direct Japanese link to Western tradition is presented most aggressively at the ffortune.net site, where the writer, identified only as 'Lumi', dismisses the idea that the Japanese are endebted to Chinese precedents. The thrust of her argument is that the planetary names came to Japan almost directly from the West, with India and China serving only as stepping stones. Bu Kong, author of the Xiuyaojing, merely relayed what he had heard from monks in the 'far west'. The use of the Chinese planetary names is simply a case of clever adaptation to Chinese culture by Bu Kong (a foreigner) and his disciple. The planetary names never really took hold in China and soon faded with the decline of Buddhism.
Lumi thus implies that the planetary names never really belonged to either the Indians or the Chinese. Bu Kong's theories, fresh from the 'far west', were picked up from his disciples within 40 years (virtually 'hot off the press') by the famous Japanese monk Kobo Daishi and brought back to Japan, where they were adopted eagerly, maintained without error for a thousand years, and are still in use today. Borrowing from the West at an early stage where the Chinese failed to do so is almost a badge of pride.
This view is understandable given that the Lumi site is about fortune-telling, for which the astrological day names are essential. However, it glosses over the historical context. Kobo Daishi went to China because it was the font of learning and civilisation at the time. Part of China's flowering was its openness to outside influences (including ideas brought in by people like Bu Kong) in a way seldom found later in history. It is strange to pretend that China is only tangentially related to the process by which the planetary names were brought back to Japan.