Introduction:
The reason why Saturday was selected for the class party is because it is a half-day.

Explanation:
There are several factors to consider when selecting a day for a class party. In this case, the main reason for choosing Saturday is because it is a half-day.

Reasoning:
1. Convenience:
- Saturday is considered to be a convenient day for a class party because it falls on a weekend. This means that students and parents do not have to worry about work or school commitments the next day.
- It allows everyone to relax and enjoy the party without the stress of waking up early the following morning or being tired during the event.

2. Scheduling:
- Saturday being a half-day means that students have more free time in the afternoon to attend the party.
- They can complete their school work in the morning and then have the rest of the day to participate in the celebrations.
- This ensures maximum participation and attendance from the students.

3. Preparation:
- Choosing Saturday as the day for the class party also allows for ample time to prepare for the event.
- Teachers and students can use the weekdays leading up to the party to organize and plan various activities, decorations, and refreshments.
- This ensures that the party is well-organized and enjoyable for everyone involved.

4. Festivals and Holidays:
- Although the given options suggest that there is a festival on Saturday, it is not a valid reason for selecting Saturday as the day for the class party.
- While it is true that festivals and holidays can sometimes coincide with Saturdays, the primary reason for choosing Saturday in this case is its half-day nature.

Conclusion:
In conclusion, Saturday was selected for the class party because it is a half-day, providing convenience for students and parents, allowing for scheduling flexibility, and providing ample time for party preparations.