My father and I always used to play tennis on Sundays.
Literal
Father [and-と] I [topic-は] Sunday [on-に] [contrast-は] always tennis [object-を] was-doing.
いつも (always) + ていた marks a habitual past action — 'used to always do.' 日曜日には stacks に (time) + は (contrast): 'on Sundays in particular.' The habitual past ~ていた is distinct from the simple past ~した — one marks a repeated pattern, the other a single event.