。
She arrived in that town last Monday.
Literal
She [topic-は] this-time-before [genitive-の] Monday [on-に] that town [to-に] arrived.
A clean polite past sentence demonstrating ~に着く ('arrive at'). The に marks the destination, and 着く is a punctual verb — the moment of arrival rather than a process. Two に particles in the sentence (Monday-に for time, town-に for destination) play different grammatical roles, an example of how Japanese gets a lot of work out of a small set of particles depending on the noun and verb context. polite ~ました closes out the past form. A workmanlike everyday narration, the kind of update you might give over the phone or in a casual report.