Sentence examples for this is once from inspiring English sources

This is once in a blue moon.

"This is once every two and a half thousand years".

This is, once more, a very Russian tragedy.

"This is once again hurting the family's core," Ms. Bogeat said.

The statistical likelihood of this is once in every 500 to 1,000 years.

This is more than a friend visiting occasionally – this is once a month more or less.

"This is once again an attack on the staff we cannot stand it any more," he said.

This is "Once and Again," ABC's show about two middle-aged brains in two -- supposedly -- "middle-aged" bodies.

This is once-in-a-lifetime food: beautiful, unusual, singularly delicious.

Beg, borrow or steal the money to go, or indeed, earn it and save up; this is once-in-a-lifetime stuff.

And this isn't frenetic Silicon Valley, this is once-sleepy Toronto.