Certain Western festivals catch on in Japan and others don’t. What is the secret factor that determines this? I would say it is a strong association with a human indulgence. Let’s consider the evidence.
Among the most successful Western festivals, I would count the following in the following order:
1. Valentine's Day
2. Xmas Day
4. St. Patrick's Day
VALENTINE's DAY seems to have had the biggest impact, because not only is there Valentine’s Day, there are effectively two Valentine’s Days – Valentine’s Day itself and White Day, one month later, when men give sweets to women. On Valentine’s Day itself only women give presents to men.
XMAS DAY is very important, but it has declined in some respects. In the past it was very much a romantic festival for young couples to get together and spend a night in a love hotel. It therefore had a sexual element, which it doesn’t have in the West. But with the decline of the younger demographic and the rise of sexual apartheid caused by "2D waifus and other forms of sexual satisfaction, it has declined although its iconography is firmly established.
HALLOWEEN continues to rise in popularity and is almost as iconically powerful as Xmas and may possibly overtake it. Halloween is focused on children, but not in an adult exclusionary way. ST. PATRICK's DAY lags far behind the other festivals, but manages to make some impact wherever there are Western style pubs and bars.
All of these festivals, it can be seen, are associated with human indulgences: VALENTINE's DAY is connected with chocolate and sex; XMAS with sex (at least in the past) and Xmas Cake, which some people like; HALLOWEEN with all kinds of sweets; and ST. PATRICK's DAY with alcohol.
Another important factor is competition/synergy with Japanese festivals. If a niche is unoccupied it can be colonized by a Western festival (Halloween, St. Valentine’s Day). Alternately it can enter into synergy with Japanese festivals.
One reason for the initial success of Xmas was its proximity to Japanese New Year, a family-centric affair that tended to exclude romantic connections. Xmas therefore took up the slack. Needless to say, this also determined its sexual character as a lover’s get-together.