、。
She skied all day that day.
Literal
She [topic-は] that day, one-day-throughout skiing [object-を] was-doing.
一日中 ('all day') is the standard expression for 'throughout the day' — 中 ('middle, throughout') after a time span gives 'all of X' (一晩中 'all night,' 一年中 'year-round'). スキーをする ('do skiing / go skiing') is the standard verbing of the loanword スキー by attaching をする — the same flexibility behind サッカーをする ('play soccer'), 散歩をする ('take a walk'). ~ていた describes the continuous activity over that time span. Skiing is huge in northern Japan, with major resorts in Nagano (host of the 1998 Winter Olympics), Hokkaido, and Niigata.