Hassaku is the abbreviation of hachigatsu sakujitsu which means August 1 on the lunar calendar.
There had been an old custom of presenting hatsuho (the first crops) to their benefactors around that time when ears of early-ripening rice plants become ripe.
For this reason, the custom is called Tanomi no Sekku (a festival for crops of the rice field.)
The word 'tanomi' in Tanomi no Sekku is written in Japanese as 田の実, which literally means crops from the rice field, but this word can also be written as 頼み, which means reliance; therefore, the custom of giving presents to people you rely on spread not only among farmers, but also among buke (samurai families) and kuge (court nobles) as a means of expressing their gratitude for the kindness of their supporters.
In and around hanamachi (fleshpot) such as Gion in Higashiyama Ward, Kyoto City, it is a traditional event conducted on August 1 (solar calendar) for geiko (young geisha) and maiko (apprentice geisha) to pay courtesy calls to ochaya (literally "teahouse" which refers to places where geisha entertain their guests) and shishotaku (residences of their masters who teach them various accomplishments).
Since August 1, 1590 (August 30, according to the Gregorian calendar) was believed to have been the day when Ieyasu TOKUGAWA officially entered the Edo-jo Castle for the first time, the Edo bakufu (the Japanese feudal government headed by a shogun) designated the day as the second most important holiday after the New Year's day.
August 1 on the old lunar calendar is also considered to be one of the three days on which it is statistically likely that a typhoon would hit Japan; the other two days are the hundred and twentieth day and the two hundred and tenth day from the first day of spring according to the lunar calendar.