I fly almost entirely domestic, so my opinion comes from these flights.

I find around Christmas and over the summer MUCH easier to upgrade. I also find flying around spring break in my home city and reading week for the universities easier.

Any time where Y is heavily loaded (all of the above) and J isn't as full of business pax (summer and Christmas), op ups have happened (although rarely for me), but more normally, I actually just stand a chance at an eupgrade!

I'd assume it's somewhat similar accross the board- if there is some holiday that would ground some business travelers, J should open up, at least a bit.