If the Zodiac Killer had a leaning towards Japanese culture, was he aware of the annual Obon festival, which is observed from the 13th to the 15th day of the 7th month. However, because of differences in the solar and lunar calendar, the 7th month is either July or August. Obon is therefore celebrated at different times in different regions, on and between these dates. This can be seen in the July 26th 1969 Stockton and Evening Record newspaper shown below, where the Buddhist holiday, born from the legend of a suffering mother "reborn into paradise", was held on August 2nd and 3rd - beginning the day after the Zodiac Killer's 408 cipher carrying the phrase "reborn in paradise" arrived at three newspapers. The Zodiac Killer's 408 cipher was mailed between July 15th and August 15th 1969, with its conception likely earlier.
Among the new alliances which this age produced were the Ikkō-ikki, a network of autonomous religious collectives that led major uprisings and controlled significant parts of Japanese society for nearly a century. Their final defeat at the hands of Japan’s warrior class set the stage both for a centralized Japanese state and the social hierarchy behind it. Nobunaga, head of the powerful Oda clan, conquered the capital city of Kyoto in 1568. In the years that followed, he made clear his intention to destroy the Honganji sect. The Patriarch Kennyo took him at his word and summoned all Ikkō-ikki from across the country to defend the faith with their lives. Despite graver circumstances than those faced by his predecessor Shonyo, Kennyo did not go as far as implying that those who fought for the Amida Buddha would be guaranteed rebirth in the Pure Land. Nevertheless, he threatened expulsion from the sect for those who failed to respond to the call and he did not crack down on those who raised the Ikkō-ikki’s now-famous banner of war: “Advance and be reborn in Paradise, retreat and fall immediately into Hell.”
These are just a few examples showing how a Zodiac Killer well-versed in Japanese culture could have taken "reborn in paradise" and used it in his 408 cipher on July 31st 1969, during the Obon featival that spanned this date. The religious connotations of "reborn in paradise" clearly came from somewhere - which may have subconsciously leaked into his early writings - and so reinforced the notion that The Mikado was a true passion of the Zodiac Killer.