。
I spent a whole day at Yoyogi Park.
Literal
Yoyogi-Park [at-で] full one-day [object-を] spent.
A simple statement. 丸 as a prefix on a time expression means 'full, whole, an entire X' — 丸一日 = 'a full day,' 丸三年 = 'a full three years.' で marks the location where an activity takes place (vs. に for static location of existence). 過ごす takes the duration with を, treating the span of time as the object being spent.