Questions & Answers about Wieczorem widzę świecący przycisk w windzie.
Wieczorem is the instrumental form of wieczór (evening), and in practice it often works like an adverb meaning “in the evening / at night (evening time)”.
- wieczór – nominative (basic dictionary form)
- wieczorem – instrumental; very commonly used for time expressions
Polish often uses the instrumental case (without a preposition) to say “in/at [part of the day]”:
- rano – in the morning (irregular form, but same idea)
- po południu – in the afternoon (here with a preposition)
- wieczorem – in the evening
- nocą – at night
You could say w wieczór, but it sounds unnatural in this kind of sentence. Wieczorem is the normal, idiomatic choice to express time here.