From star wars.com:
THIS DAY IN STAR WARS HISTORY
05.04.79
THE FAN-COINED "MAY THE FOURTH BE WITH YOU" SLOGAN GETS ITS FIRST MAJOR BOOST OF EXPOSURE WHEN THE LONDON EVENING NEWS USES IT IN A HALF-PAGE CONGRATULATORY NOTE TO MARGARET THATCHER, WHO HAD JUST WON TO BECOME BRITAIN'S FIRST WOMAN PRIME MINISTER