。
It was her habit to go to the movies on Sundays.
Literal
She [topic-は] Sunday [topic+contrast-には] movie [object-を] see [purposive-に] go [nominalizer-のが] custom was.
~のが常だ / ~のが常だった is a slightly literary expression for a habitual pattern — 'it was customary to ~,' 'it was her habit to ~.' 常 ('regular, customary') paired with the nominalizer の and the copula gives this fixed phrasing.