How would you be able to read the season of the world the player is in from the Serene seasons mod?

I'm pretty sure Serene Season's seasons are just based on gametime- the current number of in game days. If this is the case, you should just be able to find out the length of each season, and then check the current gametime to get the current season.

Since seasons will keep repeating, you'll probably also need to use some sort of remainder function. If it's been longer than one in-game year, get the remainder of dividing the current day by the year length. This way, you'll always have the correct season, regardless of whether the season has already occured.

This should totally work with commands, (assuming the seasons are based on gametime.) Gametime just keeps track of how many days have passed in the world; changing the gametime will change the season accordingly. (This is how I tested Serene Seasons awhile back to get a look at the different seasons.) The seasons are based on whatever time it currently is, not how many days have actually passed. (At least, assuming the mod hasn't changed since I last tried it.)

Assuming seasons are based on time, the seasons command probably just advances the time enough to move to the next season. (You should be able to pretty easily test this- try the seasons command, and then use /time query to get the number of days before and after.)