Yahoo Hack Day next week to reveal company's lack of a planApple's not the only Valley company planning a big event next week. Yahoo's Hack Day, a gathering for developers who want to plug their services into Yahoo's websites, will double as an unveiling of Yahoo's "open strategy." What is this strategy, exactly? An attempt to take on Google and Facebook by making it easier to tap into Yahoo's search index and user profiles. What will be announced? Nothing you haven't already heard about, we expect, but it's safe to predict you'll hear Yahoo executives begging people to build derivative search engines with its Boss service. It's a bit like Tom Sawyer asking other kids to paint the fence for him, except he forgot to bring paint. And a fence.