Hanami, sometimes prefixed with the
honorific "o-" (resulting in O-hanami, but changing the
meaning not at all) is a particular type of
party that happens in
Japan in
springtime when the
sakura are in
bloom. Any
public park worth its
salt will be covered in
falling pink petals, and if this isn't an
excuse for a party, I don't know what is.
The idea is, you get your picnic lunch packed, with lots of food and as much sake as you can afford, get your family and/or friends together, pack your portable karaoke machine, put a blanket down on top of the lovely fallen petals, and there you stay, eating, singing, and getting drunk, for as long as it's fun.
Parks tend to get very crowded during prime hanami season, but nobody really notices because Japan is inherently crowded anyway.
The word "hanami" is composed of two kanji, "hana", which means flower, and "mi", which means "look" or "see". Hence a translation of "hanami" could be "flower-viewing."