There is about one working holiday per month. In addition, there are numerous traditional, local festivals that make great footage. Long holidays include:

Long holidays
New Years
from about Dec. 29 through Jan. 3, up to the 6th
Golden Week
May 1 through 5 (in 2010), though some will only return to work May 10th
Obon (All Souls' Day)
from about August 12 to 17, when many travelers visit home or go overseas
School Holidays

for Japanese public school children are 2 weeks from the end of March to the beginning of April, when the school year starts, about a month in July/August, and a week around New Years
National Holidays (Yearly)
January 1 New Year's Day Shoot at shrines
2nd Monday of January Coming of Age Day Lots of women in kimono (in shrines)
February 11 National Founding Day
March 20 or 21
(depending on year)
Vernal Equinox
April 29 Showa no hi the former Emperor's birthday
May 3 Constitution Memorial Day
May 4 Greenery Day Many executives take the entire week off
May 5 Children's Day Also called "Boys' Day." Carp streamers.
July 20 Marine Day 3rd Monday of July
3rd Monday of September Respect for the Aged Day Not as much respect as there used to be
September 22 or 23
(depending on year)
Autumn Equinox
2nd Monday of October Health & Sports Day
November 3 Culture Day
November 23 Labor Day
December 23 Emperor's Birthday