"Of an evening" is a colloquial expression that can be taken to mean "habitually in the evening" (The Shorter Oxford English Dictionary).
The Longman dictionary says something similar.
of an evening/of a weekend etc British English: in the evenings, at weekends etc
• We often used to walk by the river of an evening.
Définitions proposed by Oxford Languages
of an evening informal • British
- on most evenings.
"he has a healthy diet consisting of cereal in the morning and meat and two veg of an evening"
- at some time in the evenings.
"you can wear these dresses of an evening or during the day"
of a morning
informal • British
- on most mornings.
"I prefer to look out the window of a morning and decide then whether a coat is needed"
- at some time in the mornings.
"he warned there would be fireworks if he was deprived of a brew of a morning"
The expression "in the evening" is much more current and, of course, not colloquial but standard ("habitually" is in most cases not needed as the context is sufficient to differentiate this meaning from the meaning "in this particular evening").

It appears that there is an error in this translation: the translation by I. Glatzer, Nahum Norbet, 1903 does not have "of" but "for", and that makes sense.
WHEN it looks as if you had made up your mind finally to stay at home for the
evening, when you have put on your house jacket and sat down after supper with a
light on the table to the piece of work or the game that usually precedes your going to
bed, when the weather outside is unpleasant so that staying indoors seems natural, and
when you have already been sitting quietly at the table for so long that your departure
must occasion surprise to everyone, when, besides, the stairs are in darkness and the
front door locked, and in spite of all that you have started up in a sudden fit of
restlessness, changed your jacket, abruptly dressed yourself for the street, explained
that you must go out and with a few curt words of leave-taking actually gone out,
banging the flat door more or less hastily according to the degree of displeasure you
think you have left behind you, and when you find yourself once more in the street
with limbs swinging extra freely in answer to the unexpected liberty you have
procured for them, when as a result of this decisive action you feel concentrated
within yourself all the potentialities of decisive action, when you recognize with more
than usual significance that your strength is greater than your need to accomplish
effortlessly the swiftest of changes and to cope with it, when in this frame of mind you
go striding down the long streets -- then for that evening you have completely got
away from your family, which fades into insubstantiality, while you yourself, a firm,
boldly drawn black figure, slapping yourself on the thigh, grow to your true stature.