Tibetan Buddhist tradition describes the intermediate state between death and rebirth, the bardo, as lasting up to forty-nine days, structured as seven periods of seven. The text most associated with this in the West is the Bardo Thodol, the Liberation Through Hearing in the Intermediate State, and the funerary practice built on it involves reading to the deceased over that period, describing what will be encountered and instructing them how to respond.
Several things about it are commonly stated wrongly.
The title Tibetan Book of the Dead is not Tibetan. It was invented by Walter Evans-Wentz for his 1927 edition, by analogy with the Egyptian Book of the Dead, and it has shaped Western reception of the text ever since, including Carl Jung's commentary and a good deal of 1960s psychedelic literature that treated it as a manual of consciousness.
The text is a terma, a revealed treasure, attributed to Padmasambhava in the eighth century but brought forward by Karma Lingpa in the fourteenth. Within the tradition that attribution is a claim about transmission rather than about authorship in the ordinary sense; historically, the fourteenth century is when the text appears.
The forty-nine days are a maximum and a ritual framework, not a fixed duration. The texts allow that rebirth may occur sooner, and that some beings pass through the bardo very quickly or not at all. Popular accounts frequently present forty-nine days as the standard length of an afterlife stage, which flattens a considerably more careful position.
Forty-nine-day observances after a death are also widespread in Chinese, Korean, Japanese and Vietnamese Buddhist practice, with rites at each seventh day.