Quoted from Bungie.net:

"For Silver Xbox LIVE Members feeling left out of the Mythic Multiplayer experience, Microsoft is gonna hook you up this weekend. Starting today, 04/10 at 9AM PST, and running through Monday, 04/13 at 9AM PST, Silver Xbox LIVE accounts will be able to play with everyone else in Halo 3's Matchmaking. If you've already got Gold, make sure you drag your Silver friends online this weekend so they can throw down with you in Mythic style. It's on Microsoft."