What is a zazenkai? "Zazen" ( 座禅) is a term you should already know. It's expressed by the Chinese characters for "seated” ("Za”) and Zen. "Kai" (会) means meeting or a coming together. So, taken together, a zazenkai is a gathering to do zazen. A less literal translation is a meeting to train intensely in Zen.
Typically, zazenkai are led by senior students. There's no set schedule or format. Zazenkai can last several hours, an entire night, or a night and a day. They might focus primarily on zazen or include other aspects of Zen training, such as okyo (chanting) or shodo (calligraphy). The choices will be up to the lead students.
Zazenkai are reserved for Daikozen-ji and Chosei Zen students who already have a regular practice but want to deepen their training and prepare for sesshin.