Seijin no Hi

I'll be 20 at the end of March next year and I really want to go the Seijin no Hi festival but would I go next January when I'm still 19 or the first January when I'm 20? Also, is it uncommon for foreigners to celebrate this holiday? I know the Japanese are kinda Xenophobic (i'm black)so if I do attend, would I get weird looks and be treated differently? I do plan to live there after school so I thought it would be nice to celebrate maturity there and then go back for good.

I do not think there's particular "festival" to celebrate for everyone who became 20 years old.
There are many ceremonies or events that is held by the local municipal government for its citizens (i.e. people registered as residents) who become 20 years old durin the school year (usually from 2nd April in the previous year till 1st April of the year). As long as these kind of ceremony/event are concerned, only the residents can participate.
People celebrate it personally, like within a family.
Some theme parks and other entertainment facilities may offer special discount off their admission, etc. for the "newly becoming 20 years old" but no so common.

Yasmine,

Most municipals in Japan send invitations to all registered residents who turn 20 on the year of the ceremony. If you fail to recieve the invitation even though you live there, you can ask your local city hall, but you need the invitation to attend the official ceremony they organize, usually at a public auditorium.

Naturally, these ceremonies work as class reunions for those who attended local junior high schools or local elementary schools, so people prefer go back to their hometown to attend the ceremony there.

Usually, the ceremony consists of speeches by people like the major or celebrities from the city, and perhaps some performances. This could be a bit of a boring event even if you understand Japanese. And then participants might go downtown to drink with their friends (although, many are not quite yet literally drinking age).

But it's a great occasion to dress up and feel yourself as part of the crowd. A lot of the youth are from different races, so I don't think you'll have any problems about it. Many municipals provide dress-up service for girls (for a price), so you may want to look into that too.

Perhaps you will make new friends your age once you start living here in Japan. Then you may want to participate in the ceremony your friends participate.

There is no Seiji-no-Hi festival.
Your local city celebrates Seijin-no-Hi meeting at city hall(normally) for about 60 mins. Urayasu City in Chiba only celebrate Seijin-no-hi in Tokyo Disney Land.

You will be invited to attend the ceremony if you are born between 2nd April 1996 and 1st April 1997 while your current residence in JP. Seiji-no-Hi normally gather former classmates of Primary, Intermediate(Junior) or High school. So, if you were born outside the city, you are welcome to attend but you have no friends there.

Each city offices give you a small gift to celebrate that you became mature person officially or legally. I got a picture frame on Seiji-no-Hi which worth around 1,000 Yen.