Question: Osechi-ryōri, are traditional food,started in the Heian Period (794-1185), sometimes used to celebrate which annual festival with its own customs?

Context paragraphs:
Tōrō: In Japan a tōrō (灯籠 or 灯篭, 灯楼 , light basket, light tower ) is a traditional lantern made of stone, wood, or metal.  Like many other elements of Japanese traditional architecture, it originated in China; however, extant specimens in that country are very rare, and in Korea they are not as common as in Japan.  In Japan, "tōrō" were originally used only in Buddhist temples, where they lined and illuminated paths.  Lit lanterns were then considered an offering to Buddha.  During the Heian period (794–1185), however, they started being used also in Shinto shrines and private homes.

Abe clan: The Abe clan (安倍氏 , Abe-shi ) was one of the oldest of the major Japanese clans ("uji"); and the clan retained its prominence during the Sengoku period and the Edo period.  The clan's origin is said to be one of the original clans of the Yamato people; they truly gained prominence during the Heian period (794-1185), and experienced a resurgence in the 18th century.  Abe is also a very common Japanese surname in modern times, though not everyone with this name necessarily is descended from this clan.

Heian Palace: The Heian Palace (平安宮 , Heian-kyū ) or Daidairi (大内裏 ) was the original imperial palace of Heian-kyō (present-day Kyoto), the capital of Japan, from 794 to 1227.  The palace, which served as the imperial residence and the administrative centre of for most of the Heian period (from 794 to 1185), was located at the north-central location of the city in accordance with the Chinese models used for the design of the capital.

Japanese New Year: The Japanese New Year (正月 , Shōgatsu ) is an annual festival with its own customs.  Since 1873, the official Japanese New Year has been celebrated according to the Gregorian calendar, on January 1 of each year, New Year's Day (元日 , Ganjitsu ) .  However, the celebration of the traditional Japanese New Year is still marked on the same day as the contemporary Chinese, Korean, and Vietnamese New Years.

Early Middle Japanese: Early Middle Japanese (中古日本語 , chūko nihongo ) is a stage of the Japanese language used between 794 and 1185, a time known as the Heian Period.  It is the successor to Old Japanese.  It is also known as Late Old Japanese, but the term "Early Middle Japanese" is preferred, as it is closer to Late Middle Japanese (after 1185) than to Old Japanese (before 794).

Japanese poetry: Japanese poetry is poetry of or typical of Japan, or written, spoken, or chanted in the Japanese language, which includes Old Japanese, Early Middle Japanese, Late Middle Japanese, and Modern Japanese, and some poetry in Japan which was written in the Chinese language or "ryūka" from the Okinawa Islands: it is possible to make a more accurate distinction between Japanese poetry written in Japan or by Japanese people in other languages versus that written in the Japanese language by speaking of Japanese-language poetry.  Much of the literary record of Japanese poetry begins when Japanese poets encountered Chinese poetry during the Tang dynasty (although the Chinese classic anthology of poetry, "Shijing", was well known by the literati of Japan by the 6th century).  Under the influence of the Chinese poets of this era Japanese began to compose poetry in Chinese "kanshi"); and, as part of this tradition, poetry in Japan tended to be intimately associated with pictorial painting, partly because of the influence of Chinese arts, and the tradition of the use of ink and brush for both writing and drawing.  It took several hundred years to digest the foreign impact and make it an integral part of Japanese culture and to merge this "kanshi" poetry into a Japanese language literary tradition, and then later to develop the diversity of unique poetic forms of native poetry, such as "waka", "haikai", and other more Japanese poetic specialties.  For example, in the "Tale of Genji" both "kanshi" and "waka" are frequently mentioned.  The history of Japanese poetry goes from an early semi-historical/mythological phase, through the early Old Japanese literature inclusions, just before the Nara period, the Nara period itself (710 to 794), the Heian period (794 to 1185), the Kamakura period (1185 to 1333), and so on, up through the poetically important Edo period (1603 and 1867, also known as "Tokugawa") and modern times; however, the history of poetry often is different than socio-political history.

History of Japan: It is widely accepted that first human habitation in the Japanese archipelago can be traced back to prehistoric times.  The Jōmon period, named after its "cord-marked" pottery, was followed by the Yayoi in the first millennium BC, when new technologies were introduced from continental Asia.  During this period, in the first century AD, the first known written reference to Japan was recorded in the Chinese "Book of Han".  Between the fourth century and the ninth century, Japan's many kingdoms and tribes gradually came to be unified under a centralized government, nominally controlled by the Emperor.  The imperial dynasty established at this time continues to reign over Japan to this day.  In 794, a new imperial capital was established at Heian-kyō (modern Kyoto), marking the beginning of the Heian period, which lasted until 1185.  The Heian period is considered a golden age of classical Japanese culture.  Japanese religious life from this time and onwards was a mix of Buddhism, and native religious practices known as Shinto.

Classical Japanese language: The classical Japanese language (文語 "bungo", "literary language"), also called "old writing" (古文 "kobun"), is the literary form of the Japanese language that was the standard until the early Shōwa period (1926–89).  It is based on Early Middle Japanese, the language as spoken during the Heian period (794–1185), but exhibits some later influences.  Its use started to decline during the late Meiji period (1868–1912) when novelists started writing their works in the spoken form.  Eventually, the spoken style came into widespread use, including in major newspapers, but many official documents were still written in the old style.  After the end of World War II because of the surrender of Japan, most documents switched to the spoken style, although the classical style continues to be used in traditional genres, such as haiku and waka.  Old laws are also left in the classical style unless fully revised.

Osechi: Osechi-ryōri (御節料理 or お節料理) are traditional Japanese New Year foods.  The tradition started in the Heian Period (794-1185).  "Osechi" are easily recognizable by their special boxes called "jūbako" (重箱), which resemble "bentō" boxes.  Like "bentō" boxes, "jūbako" are often kept stacked before and after use.

Heian period: The Heian period (平安時代 , Heian jidai ) is the last division of classical Japanese history, running from 794 to 1185.  The period is named after the capital city of Heian-kyō, or modern Kyōto.  It is the period in Japanese history when Buddhism, Taoism and other Chinese influences were at their height.  The Heian period is also considered the peak of the Japanese imperial court and noted for its art, especially poetry and literature.  Although the Imperial House of Japan had power on the surface, the real power was in the hands of the Fujiwara clan, a powerful aristocratic family who had intermarried with the imperial family.  Many emperors actually had mothers from the Fujiwara family.  "Heian" (平安 ) means "peace" in Japanese.

Supporting paragraphs: Osechi Japanese New Year
Answer: Japanese New Year