Just what it says on the tin; ability to configure mobs spawn based on season/subseason.

Some applications:

• increased insect spawnrate in summer
• thermal foundation ice blazes in winter
• seasonal hostile mobs
• actively spawn certain mobs (rabbits, deer, et cetera) in spring/summer only
• baby animals in spring
  etc

Check here there should already be compatibility in the mod on the wiki under Conditions just above halfway down the page it says this:

Conditions that are present with Serene Seasons:

winter:
summer:
spring:
autumn:

Not sure about baby animals, but it may be possible with nbt {Age:#}
Do note that the age of the animal must start below 0 to be a baby, and when their age reaches 0 they grow up.

note- I am not the mod creator and I am not sure what version this is

I suggested this before that was implemented (note the date on the issue) and was very happy when I saw it in a changelog. Thank you, though!