When!
You must account for chronology when writing history papers. Do not be afraid to use dates. History never makes sense when you don't know the sequence of events. If you are trying to show cause-and-effect, for example, it is important for the reader to see that the cause preceded the effect. If the events you are describing are not arranged chronologically, then the logic of your arrangement of them should be clear to the reader. The following history-paper writing advice comes from the Hamilton College website:
Watch the chronology. Anchor your thesis in a clear chronological framework and don't jump around confusingly. Take care to avoid both anachronisms and vagueness about dates. If you write, “Napoleon abandoned his Grand Army in Russia and caught the redeye back to Paris,” the problem is obvious. If you write, “Despite the Watergate scandal, Nixon easily won reelection in 1972,” the problem is more subtle, but still serious. (The scandal did not become public until after the election.) If you write, “The revolution in China finally succeeded in the twentieth century,” your professor may suspect that you haven’t studied. Which revolution? When in the twentieth century? Remember that chronology is the backbone of history. What would you think of a biographer who wrote that you graduated from Hamilton in the 1950s?