Why might it be fortuitous for a tall, dark man to enter your house early tomorrow, January 1st?

A popular belief says the first person who enters your home represents and influences the kind of year you'll have. If a tall, dark-haired man enters your house on the New Year, it is considered particularly fortuitous. This visitor should be greeted warmly and treated generously.

It is even better if he brings a gift. Especially lucky items are a silver coin, a lump of coal, a piece of bread, etc. Blondes and redheads (men and women), as well as women of any hair color, are all said to bring bad luck if they enter your home first.

http://festivals.iloveindia.com/new-year/superstitions.html

This belief comes from the celebrations of Hogmanay in Scotland:

"First footing" (the "first foot" in the house after midnight) is still common in Scotland. To ensure good luck for the house, the first foot should be male, dark (likely a reference to the Viking days, when blond strangers arriving on your doorstep meant trouble) and should bring symbolic coal, shortbread, salt, black bun and whisky.

http://www.rampantscotland.com/know/blknow12.htm