It needs to be outside!! It will be perfectly happy outdoors - that is its preferred location - and will withstand plenty of cold. You just need to take precautions against the soil in the container freezing, so wrap or somehow protect the container if temperatures should drop much below freezing.

Ideally, you should plant it in the ground as soon as you can work the soil. Whoever told you that Mother's Day nonsense is nuts!!

It's in the 30's here at night and low to mid 40's during the day and will be for the rest of the month or so and I just purchased two small containerized JM's from a local nursery and they are happily waiting outside until I can plant them properly!

The tree needs to be kept outside so it stays dormant. Make sure the 5 gallon bucket has holes in the bottom so the water can drain through. If you're not planting the tree yet as a precaution to protect the tree from the cold I would dig a hole and bury the bucket in the ground.

I agree, it should be outside all year. Mother's Day is when you plant vegetables in a cold winter climate. I have Japanese Maple bonsai that have survived down to 10F in shallow pots outdoors.