Sorry I didn't specify. You only receive one maingate per year, so if you got one on your CP, that's it--and you would use the seasonal ID with it.
What you mentioned only matters when the next calendar year rolls around. They send it after you work a shift, an incentive to work as soon as possible and to avoid sending you something for nothing.
I went from CP to Seasonal around this time frame and did not receive a new maingate until I worked in January.
CP and Seasonal are very similar statuses and use the same maingate passes, which I'm guessing is why it is done that way.