The full moon that falls in July in most years is one of the most consequential dates in the Buddhist calendar. Asalha Puja, kept across the Theravada world, commemorates the Buddha's first sermon in the deer park at Sarnath, the turning of the wheel of the teaching, and is often called Dhamma Day for that reason. The day after it begins Vassa, the three month rains retreat, when monks stay within their monasteries for the season of the monsoon, the period from which Western writers took the phrase Buddhist Lent and at whose end the kathina robe offerings are made.
The same full moon, reckoned in the Hindu month of Ashadha, is Guru Purnima, the day of teachers, on which students honour their gurus and tradition honours Vyasa, compiler of the Vedas, as the first of them. The observance crosses traditions: Jains keep the day for Mahavira's first disciple, and Buddhists in India share it with the Theravada commemoration, so the one moon carries the teacher's festival for three religions at once.
The month's fixed dates come from newer calendars. Tanabata, the Japanese star festival of the seventh of July, descends from the Chinese Qixi legend of the weaver girl and the cowherd, the stars Vega and Altair allowed to meet across the river of the Milky Way one night a year; wishes are written on paper strips and tied to bamboo. And the ninth of July is the Martyrdom of the Bab, when Baha'is worldwide suspend work at noon to mark the execution of the Bab by firing squad in Tabriz in 1850.
Sources Theravada Buddhism: A Social History from Ancient Benares to Modern ColomboRichard Gombrich with Hindus: Their Religious Beliefs and PracticesJulius Lipner, A Concise Encyclopedia of the Baha'i FaithPeter Smith