I think it is at levels when a spontaneous caster gets the first spell known of a at least 2nd level. So for sorcerers it is at levels 4, 6, 8, 10, 12, 14, 16, 18 and for bards it is at levels 4, 7, 10, 13, 16.
... Now I wonder if it is at every other level (every third) level instead?