The [i][b]Sword of Crota[/b][/i] mission will always have at least 4 majors - the [b]Swarm Princes[/b], who are the bosses of the mission. If you activate the mission on [i]Heroic[/i] then more majors are likely to spawn but you can just do the normal difficulty story and you'll get 4 majors. Apart from that, there are always 2 major nights right outside the temple in Archer's Line (the area you spawn in on the Moon for [i]Patrol[/i]), and there are 2 more down in the [b]Circle of Bones[/b] by the entrance to the [b]Summoning Pits[/b]