For once is used to emphasize that something happens on this particular occasion, especially if it has never happened before, and may never happen again.
[emphasis]
For once, dad is not complaining.
His smile, for once, was genuine.
See full dictionary entry for once
for once in British English
this time, if (or but) at no other time
See full dictionary entry for once
for once in American English
for at least one time
See full dictionary entry for once
Examples of 'for once' in a sentence
for once
We have spoken and, just for once, we will have our way.
The Sun (2017)
For once the reality may come close to matching the rhetoric.
Times, Sunday Times (2015)
For once that optimism may not be misplaced.
Times, Sunday Times (2013)
But for once the hype may be justified.
Times, Sunday Times (2010)
If the party listens to the likes of them it might be worth voting for once again.
The Sun (2015)
Congratulations for once again exposing the greed of the upper tiers of society.
Times, Sunday Times (2012)
And just for once there was no ensuing row.
The Sun (2015)
For once, this may be true.
Times, Sunday Times (2009)
For once, he may have timed his run just right.
Times, Sunday Times (2013)
For once again he was waiting for Perpetual.
Christianity Today (2000)
And will Imogen cotton on to what is happening for once in her life?
The Sun (2006)
He was pleased to win, but for once he was not overwhelmed by his own brilliance.
Times, Sunday Times (2008)
I hope just for once he will win.
The Sun (2015)
Just for once she had a significant personal role in his life, and her energy and confidence rose.
Times, Sunday Times (2011)
Can Arsenal win ugly for once?
Times, Sunday Times (2010)
Just for once I tried to listen instead of doing all the talking.
Frankie Dettori with Jonathan Powell FRANKIE: The Autobiography of Frankie Dettori (2004)
Just for once, let's hear it for the man in the street.
Times, Sunday Times (2007)
Just for once, it was her husband's show.
Times, Sunday Times (2006)
He was pure slapstick and for once Mr Tumble just watched him and got to be quite serious!
The Sun (2010)
But just for once, out on the course, there is a little more activity than usual.
Times, Sunday Times (2011)
Just for once, should we not be honest, accept our part in an unfolding disaster and do something about it?
Times, Sunday Times (2014)
The Greeks are angry with the Germans for once again in effect telling them how to levy taxes and to organise their economy.