Moon Viewing Party – O-tsukimi or Tsukimi

Tsukimi (月見?) or Otsukimi, literally moon-viewing, refers to Japanese festivals honoring the autumn moon. The celebration of the full moon typically takes place on the 15th day of the eighth month of the traditional Japanese lunisolar calendar; the waxing moon is celebrated on the 13th day of the ninth month. These days normally fall in September and October of the modern solar calendar.

The tradition dates to the Heian era, and is now so popular in Japan that some people repeat the activities for several evenings following the appearance of the full moon during the eighth lunisolar month.

Tsukimi traditions include displaying decorations made from Japanese pampas grass (susuki) and eating rice dumplings called Tsukimi dango in order to celebrate the beauty of the moon. Seasonal produce are also displayed as offerings to the moon. Sweet potatoes are offered to the full moon, while beans or chestnuts are offered to the waxing moon the following month. The alternate names of the celebrations, Imomeigetsu (literally “potato harvest moon”) and Mamemeigetsu (“bean harvest moon”) or Kurimeigetsu (“chestnut harvest moon”) are derived from these offerings.

some information can be found here, https://en.wikipedia.org/wiki/Tsukimi

Additional resources:
http://www.inhamamatsu.com/recommend/cat2/9/moon-viewing-party.php
http://japanesefood.about.com/od/holidaytraditionalfood/a/tsukimifood.htm
http://emojipedia.org/moon-viewing-ceremony/
https://asianinspirations.com.au/asian-culture/japanese-moon-festival-legend/
Celebrate: Tsukimi (Moon-Viewing)
http://kikuko-nagoya.com/html/tsukimi.html
http://www.kyotoguide.com/ver2/thismonth/moon10.html

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An anime that this appears in is XxxHolic. Yuko is on the back porch with black mokona and they are drinking sake by the moon light and she mentions that they are having a moon viewing party.