Use Instrumental to express 'in/at' a time of day.
To express 'in the morning/afternoon/evening' and 'at night' you use the instrumental of џтро, дљнь, вљчер, and нћчь, just as with seasons of the year.
џтром 'in the morning' днём 'in the afternoon'
вљчером 'in the evening' нћчью 'at night'
The form днём means either 'in the daytime' or 'in the afternoon'.
The word нћчь means 'nighttime', i.e. the time when people normally sleep, and does not refer to the hours of dark before bedtime, for which вљчер is used. Thus, if you used нћчь in the expression for 'What are you going to do tonight?' it would sound rather strange, because 'Sleep' would be the only answer under normal circumstances. Вљчер begins at sunset or after the evening meal.
Instead of the word for 'this' or 'last' to express 'this morning (afternoon, evening)' or 'last evening (night),' use сегћдня 'today' and вчерђ 'yesterday'.
вчерђ џтром yesterday morning сегћдня џтром this morning зђвтра џтром tomorrow morning
вчерђ днём yesterday afternoon сегћдня днём this afternoon зђвтра днём tomorrow afternoon
вчерђ вљчером last evening сегћдня вљчером this evening зђвтра вљчером tomorrow evening
вчерђ нћчью last night сегћдня нћчью tonight зђвтра нћчью tomorrow night