2018, 2019, and 2020 the Marvel Day At Sea, and Star Wars Day At Sea sailings have been in January and February (and on a few occasions very early march)
My educated guess would be the first 9 sailings for Western Caribbean 7 Nights from Canaveral, and the first 9 sailings for 5 Night Western Caribbeans from Miami, in 2021, will be the trend for at least one more year.
2022 we might see a change in this pattern as it will be the first year that will have a 5th ship in rotation with the launch of the Disney Wish in late 2021.