The phrase "a period when a" is correct and usable in written English.
It can be used to describe a specific timeframe during which an event or situation occurs.
Example: "The 1960s was a period when a significant cultural revolution took place in the United States."
Alternatives: "an era in which a" or "a time during which a".
"It was 1979," Guest remembers, "a period when a lot of people were high a lot.
Borrowers are not liable for interest relating to a period when a lender has not provided the information.
She's coming into midcareer, a period when a style can, without vigilance, turn into a set of moves.
In a period when a modernist aesthetic prevailed, with its deep suspicion of narrative in painting, her work could appear too confidently representational, too middlebrow.
This, moreover, was a period when a sense of what revolution meant was itself revolutionary, and, for many ordinary people, transforming.
Because of revisions during previews, the press opening was postponed several times, into a period when a lot of the houses had been pre-sold to theater parties.
Some of the environmental groups have accused Duke of coming in with the pipeline proposal during a period when an energy shortage in California has caused rolling blackouts, and there is worry about a similar situation in the Northeast this summer.
For some time, it was held that Celtic stood in an especially close relation to the Italic branch; some scholars even spoke of a period when an Italo-Celtic "nation" existed, toward the end of the 2nd millennium bc.
Materiel was also escorted across military checkpoints by security personnel working for the Accused… [playing] a vital role in the operations of the RUF/AFRC during a period when an international arms embargo was in force..
But the definition of a bear market is a period when prices fall a long way.
