。
Mother bought me two pairs of pants last Sunday.
Literal
Mother [topic-は] this before [gen-の] Sunday [on-に] pants [obj-を] 2 [counter-本] bought-for-me (polite).
The counter 本 applies to long thin objects, and pants (ズボン) count as one — a useful reminder that counter choice follows the shape/category of the object rather than English intuition. ~てくれました marks the action as done for the speaker's benefit, here in polite ~ました form. に marks the time (日曜日) when the action happened.