。
It was her habit to go to that museum on Sundays.
Literal
She [topic-は] Sunday [topic+contrast-には] that museum [to-に] go [nominalizer-のが] custom was.
~のが常だった is a slightly literary expression for a habitual past pattern: 'it was customary, it was her habit.' 常 ('regular, customary') paired with the nominalizer の and the past copula gives this fixed phrasing — common in retrospective biographical or narrative writing. 日曜には rather than 日曜日には is a slightly more colloquial drop of the redundant 日; both are used naturally.