Meaning: The friends' happiness made the whole day feel happy and joyful.
Choice of Words: Happiness is a feeling of great pleasure and contentment. Filled means to make something full.
Alternative Expressions
The day was filled with happiness.
Meaning: This sentence also means that the feeling of happiness was present throughout the entire day.
Choice of Words: The sentence structure is slightly different from the first example, but the meaning is the same. "The day" is the subject of the sentence, and "was filled with happiness" is the predicate.
Happiness permeated the day.
Meaning: This sentence means that the feeling of happiness was present in every part of the day.
Choice of Words: The word "permeated" means to spread or diffuse throughout something. In this sentence, it is used to describe how happiness spread throughout the entire day.