。
That year, she had the entire summer off.
Literal
She [topic-は] that year, one-summer all [subject-が] vacation was.
その年 ('that year') is a time-frame adverbial without a particle — Japanese routinely drops the time-に when context makes it clear. ひと夏全部 ('all of one summer') uses ひと~ as a counter-prefix meaning 'one whole' (cf. ひと月 'one month,' ひと冬 'one whole winter'). The が marks the duration as the (focused) subject of 休みだった ('was vacation/break'). The whole sentence has a slightly nostalgic flavor — a year associated with a long, free stretch of time, the kind of formative summer often invoked in Japanese coming-of-age fiction.