When Shadow is trying to tell Wednesday about his dream with the thunderbirds, Wednesday gets quite angry and then says they're heading to San Francisco:
"I dreamed of thunderbirds...," said Shadow. "And a tower. Skulls..." It seemed to him very important to recount his dream.
"I know what you were dreaming. Everybody damn well knows what you were dreaming. Christ almighty. What's the point in hiding you, if you're going to start to fucking advertise?"
Shadow said nothing.
There was a pause at the other end of the telephone. "I'll be there in the morning," said Wednesday. It sounded like the anger had died down. "We're going to San Francisco. The flowers in your hair are optional." And the line went dead.
American Gods: The Tenth Anniversary Edition, chapter 11
What does the line "the flowers in your hair are optional" mean? I presume it has something to do with the fact that Shadow was "advertising" where he was, but I'm not quite sure exactly what this is supposed to be implying.