Kannazuki (Kaminashizuki) is an another name for October (lunar calendar) in Japan. Today, it is also used as an another name for October in the solar calendar.
Although there are several views on the origin of the word 'Kannazuki (神無月)' (refer to October), ' 神無' is a Chinese character used as a phonetic symbol rather than for its meaning. But because the term is written with the Chinese characters "神無" (lit. "there are no gods"), it is widely believed to be a month during which the deities disappear from the land. In other words, during the month of October all the regions of Japan other than Izumo are emptied of their gods, because all the gods in the entire land assemble at Izumo Taisha Shrine to discuss the year's events. In fact, October in the lunar calendar is called Kamiarizuki (literally a month when deities are around) in Izumo Province. Consequently, the etymological explanation of the term as coming from "the month of no gods" has become quite deep-rooted. Yet in reality, Izumo Taisha Shrine onshi (priests) have been actively trying to spread this interpretation throughout the country since the medieval period, despite the fact that the term actually arose when the Chinese characters "神無" were used phonetically to replicate the sound--not the meaning--of "kaminashi."
Various views surrounding 'Kannazuki'
It is generally believed that deities gather at Izumo Taisha Shrine to discuss matchmaking. Consequently, in Sado Province there used to be a local custom of avoiding any marriage proposals during the tenth month. Furthermore, in Kitakyushu the custom used to be for unmarried men and women to seclude themselves in shrines on the days when the gods departed for and returned home from Izumo. Another belief holds that only Kunitsukami--deities belonging to Okuninushi line--go to Izumo, as does yet another theory according to which Amatsukami, including Amaterasu Omikami (the sun goddess), join them in going to Izumo (there is also an oral tradition which claims that the Amaterasu enshrined at Amaterasu-jinja Shrine in Tsushima is the last to arrive and the first to depart among the gods that visit Izumo during Kannazuki). It was amidst such theories that the concept of a "Rusugami," or "god who remains home," was developed. Ebisu is most often thought to be a Rusugami, though some regions celebrate Ebisuko, or gatherings at which people worship Ebisu, during the tenth month.
In Izumo, Shinto rituals associated with "Kamiarizuki" are held during the tenth month--according to the lunar calendar--at Izumo Taisha Shrine as well as several other shrines. The Kamimukae-sai Festival, a festival to welcome the gods who have gathered together from all over the country, is held on the night of October 10 (under the lunar calendar) at Inasahama where, according to the mythical accounts given in the Kojiki and Nihonshoki, Kuniyuzuri took place.
Following this, the Kamiari-sai Festival is held from October 11 thru 17 (according to the lunar calendar) based on the belief that the gods are holding a meeting at Izumo Taisha Shrine during this period. On October 18 of the lunar calendar, the 'Karasade-matsuri Festival,' a festival to bid farewell to the deities, is held in the haiden (a building for worshipping deities) of Izumo Taisha Shrine. Juku-sha Shrine, thought to be where the gods stay during their visit, is located inside the hedges of Izumo Taisha Shrine. Other than Izumo Taisha Shrine, the Kamiari-sai Festival is held at Honomisaki-jinja Shrine (Taisha-cho, Izumo City), Asayama-jinja Shrine (Asayama-cho, Izumo City), Mankusen-jinja Shrine (Hikawa-cho), Kanbara-jinja Shrine (Unnan City), Sata-jinja Shrine (Kashima-cho, Matsue City), Mezuki-jinja Shrine (Saika-cho, Matsue City), Kamosu-jinja Shrine (Oba-cho, Matsue City) and Taga-jinja Shrine (Matsue City) (Asakumi-cho, Matsue City).