Here's why:

* Simile: A simile uses words like "like" or "as" to compare two things. For example, "The sun is like a giant ball of fire."

* Metaphor: A metaphor directly equates two things without using "like" or "as." "August is the day of sun" uses this direct comparison, equating August with the intense heat of the sun.

Therefore, this phrase is a metaphor, not a simile.