This is the 9492nd most frequent Polish word.
wieczór
Evening.
Here, 'wieczór' is used as a noun referring to the period of time after the day and before night.
Wieczór jest moją ulubioną porą dnia.
Evening is my favorite time of the day.
In this sentence, 'wieczór' is used in the instrumental case 'wieczorem' to indicate the time at which the action will occur.
Spotkamy się wieczorem.
We will meet in the evening.
Here, 'wieczorny' is the adjective form derived from 'wieczór', describing something related to the evening.