When the plants need bigger pots or fresh soil. This will vary with the individual specimens.

The above advice is reasonable with regards to the frequency of repotting but not so much as to the timing of repotting :-)

If there is much disturbance of the root system anticipated - a need for root pruning, a need to loosen the root system due to congestion or circling roots, removal of old soil media - then it is best to wait until the plant is fully dormant, like midwinter. I would certainly follow that schedule with regards to the JM's and conifers. If you are just up-sizing - moving into a larger container with minimal to no root disturbance - then this specific time is not quite so critical.

Generally, any potting activity is best accomplished when the plant in question is not in full growth.

Pot up any time, but potting up while the plant is in active growth is best because "over-potting" is less likely to be a player. Keep in mind that potting up is a sort of half measure and ensures the plant will always be limited by congestion in the original root mass. Repotting eliminates the limitations associated with root congestion because root congestion is (or should be) relieved during each repotting session and the plant given a fresh change of soil - hopefully something that allows you to water appropriately.

Temperate plants that lose their leaves, conifers other than mugo pine ( mugo in middle of summer), and broad-leaf evergreens are best repotted in spring unless there is no danger of frost injury to freshly grown roots over the winter, then fall after the plant is dormant or quiescent works. The deciduous material where it's colder should be repotted just before or at the onset of budswell. Conifers follow the deciduous material by 2-3 weeks. Houseplants and most succulents are best repotted in the month prior to their most robust growth period. For most of the country, that means June, and Father's Day is easy to remember because it comes just before the summer solstice when these plants are at their strongest and recovery from the work will be fastest - early June for the deep south.

Al