In Mordenkainen's Tome of Foes, the Eladrin race has Winter, Summer, Spring, and Fall seasons. I know it affects the Fey Step, but how?
Also, could you make an eladrin without choosing a season?
For the MtoF Eladrin, the relevant traits can be found on page 61 and 62. These include the fact that choosing a season is mandatory:
Choose your eladrin's season: autumn, winter, spring, or summer.
As you can see, there is no option for having no season. You simply have to choose one. Do note that you may be allowed to change the associated season on a long rest. On page 61, it is stated that:
When finishing a long rest, any eladrin can change their season
How the season affects their fey step ability is on the right side of page 62:
When you reach 3rd level, your Fey Step gains an additional effect based on your season
This quote is followed by a description of how each season affects the Fey Step in a different way.
Note that there is also a completely different and unrelated version of the Eladrin presented in DMG page 286 as an example of creating custom subraces. This version does not use seasons (or even mention them as far as I could tell) and has a different take on Fey Step.
1. Mention suggested by David Coffron