Using a coffee grinder or mortar and pestle finely grind annatto seeds. Combine ground annatto seeds with salt, pepper, oregano, cumin and allspice in a bowl. Rub the spice mixture into the pork, place in a large resealable plastic bag and refrigerate overnight. The following day in a skillet or comal toast the cinnamon sticks over medium-high heat 2 minutes on each side, place in a 4-5 slow cooker along with pork, garlic, grapefruit juice, orange juice, lime juice and bay leaf. Cover and cook on low for 6-8 hours, until meat is tender. Remove pork from slow cooker, shred, and drizzle with juice and serve warm with tortillas, use for tortas or sopes.