Field day or Undoukai (運動会) which can also be called Taiiku-sai (体育祭) is another key event in school life anime. It’s an all-school event where classes are typically divided into the white team and red team.
Competitions are held in the track including Tamaire (玉入れ) which is seeing which team can throw the most balls in a basket, Kibasen (騎馬戦) which is where 3 people create a horse below and one person goes on top to try to take the bandana or hat off the opposing team.
The clear highlight is at the climax of the event where the two teams battle it out in a relay run.
A lot of drama can happen here where the heroine may shout out to cheer for the main characters name, or he may faint during or after the race to wake up in the nurses office to find a fellow classmate holding his hand with tears in her eyes.
It is another event through the school-year and can be a big deal for many students (in the anime world) as not only is it competitive, it is an opportunity to show-off to the people they admire, attract new fans, show a side of a character that wasn’t evident before, and can be used as a stage for a lot of drama. Even if the main character isn’t some hidden athlete that that catches the class by surprise, the person they admire may feel attracted to another character thereby sending the main character in despair (and then motivate him to secretly train).
There are many anime that have Undoukai episodes, and even whole series that feature the concept of Undoukai like Battle Athletes Daiundoukai.