YGB,
Despite what some other posters have written, what you propose is possible. If a K1 leaves the USA before marriage the consulate can re-issue the visa. The person will have to be back in the USA and get married within 90 days of the original entry - before late July in your case.
But, the consulate is not required to re-issue the visa. So while what you propose is possible it is not a sure thing. The safest thing to do would be what John & Annie suggest.
Yodrak
QUOTE(YGB @ Apr 1 2007, 11:26 PM)

Hello all,
My fiancee received her K1 visa about a month ago and is planning on moving here (U.S.) at the end of April. We are planning on getting married in June.
The problem is she wants to go back to Japan for her brother's wedding in early June before we get married.
Is this possible?