8 events the sources record for this year, grouped by region.
What happened
Japan (8)
In the first month, the calendar official Ōkasuga no Manomaro asks to make a new calendar based on the Gokireki (Five Cycles) method, because the Daien calendar differs from the Tang calendar; granted. (Nihon Montoku Tennō Jitsuroku 日本文德天皇實錄 卷第八 「天安元年」 1月)
In the second month, on day 丁亥, Fujiwara no Yoshifusa is appointed Daijō-daijin, Minamoto no Makoto Sadaijin and Fujiwara no Yoshimi Udaijin. (Nihon Montoku Tennō Jitsuroku 日本文德天皇實錄 卷第九 「天安元年」 2月)
In the second month, on day 己丑, the era is changed to Ten'an, citing a white deer of Mimasaka and a connected-branch tree of Hitachi as omens. (Nihon Montoku Tennō Jitsuroku 日本文德天皇實錄 卷第九 「天安元年」 2月)
In the third month, on day 癸丑, guards of the left and right, the Hyōe and kebiishi are sent to capture bands of robbers south of the capital. (Nihon Montoku Tennō Jitsuroku 日本文德天皇實錄 卷第九 「天安元年」 3月)
In the fourth month, on day 庚寅, three barriers are newly established in Ōmi at Ōsaka (the old barrier), Daiseki and Yūka, with provincial officials and militia stationed there. (Nihon Montoku Tennō Jitsuroku 日本文德天皇實錄 卷第九 「天安元年」 4月)
At the end of day 乙丑 in the fifth month, the rain does not clear, the floods overflow, cutting roads and bridges and breaking the embankments (on day 丙辰 of the same month, water also overflows in the capital). (Nihon Montoku Tennō Jitsuroku 日本文德天皇實錄 卷第九 「天安元年」 5月)
In the sixth month, on day 庚寅, Dazaifu reports by post horse that about 300 men, including the Kamigōri officials Urabe no Kawachimaro and the Shimogōri Gitairyō Atai no Uranushi, surrounded the residence of Governor Tachino Masamine and killed Masamine with fire arrows, together with ten of his attendants and six sakimori (frontier guards). (Nihon Montoku Tennō Jitsuroku 日本文德天皇實錄 卷第九 「天安元年」 6月)
In the seventh month, on day 辛亥, Dazaifu is ordered by edict to pardon the wives and children of innocent men who were forced to join the Tsushima rebels or died in prison. (Nihon Montoku Tennō Jitsuroku 日本文德天皇實錄 卷第九 「天安元年」 7月)