(my off-season clothes reside in the canvas boxes and
hanging garment bag in my small coat closet)
Mid September and it's time to plan for the season switch. Since many clothes are appropriate for all-year round, you should be able to transition easily while you find the time to do it.
The first step is to clean everything before packing it away whether tossing it into the wash or sending it out to the dry cleaner. This not only prevents clothes moths (moths are attracted to fabrics soiled by sweat, skin, hair, food, etc), but to help unseen stains from setting. And don't forget to get any shoes repaired so they're ready to go when the time comes back around.
When you do pack things away, it's very easy to over-edit so be careful. Remember! When in doubt, keep it...
Here are some great storage options, and these clear boxes are perfect for under the bed if you don't have closet space.