When is it appropriate to use 在 with a time phrase?
Yes, you can

他去年去了中国度假。= He went to China for a holiday last year.
他在去年去了中国度假。= He went to China for a holiday in last year.

The first sentence stated a fact '他去了中国度假', and '去年' is the time expression that add additional information on what time it took place
The second sentence '在' emphasize the time period '去年'; &quot;去了中国度假&quot; took place &quot;in last year&quot; along with other things.
The follow example may illustrate this emphasis better

&quot;他去年死了&quot; = &quot;He died last year&quot; ('he died' is the fact; 'last year' is a relative phrase that add additional information to it, &quot;when did he die? Last year&quot;)
&quot;他在去年死了&quot; = &quot;He died in last year&quot; ('in last year' is an important fact, not two year ago, not three years ago)