八月十五月月饼 seems to be a playful phrase mixing up words related to the Mid-Autumn Festival、Let's break it down:

八月十五 (bā yuè shí wǔ) means the 15th day of the eighth month in the lunar calendar, which is when the Mid-Autumn Festival occurs.
月饼 (yuè bǐng) means mooncake, a traditional food eaten during the Mid-Autumn Festival.

A more conventional way to express this might be: 八月十五吃月饼 (bā yuè shí wǔ chī yuè bǐng), which means eating mooncakes on the 15th day of the eighth month.