It is interesting to see how different people find meaning in a question like this. My interpretation of the question is a bit more literary, I guess, because what comes to my mind is connected to the kinds of historical events that can led a positive tone to a work of literature.
There are many works of literature that contain authentic historical details, and these can, of course, make a work more positive or negative. Imagine, for example, a novel that begins with the final spike driven in the transcontinental railroad. Such an event would lend great optimism to any novel. Similarly, a novel that included the tumbling of the Berlin Wall would be driven in a positive way by that event. The end of World War II would be another example of an event that would make a story more positive. A more recent event, the election of President Obama, would certainly make a story about race relations in the United States more positive.
On the other hand, there are plenty of negative historical events for an author to use in a story. Some examples are the Holocaust, the attack of 9/11, the Cold War, or the Depression.
Generally, when an author includes historical details in a work of fiction, he or she is certainly aware that such events help to set the tone for a story, just as the setting of a story helps to set the tone. One good example of a story with rich historical details is Ragtime, by E. L. Doctorow.
I hope someone is able to provide an answer that is of use to you. Good luck!
Naturally, an answer to such a wide and open question will yield many wide and open answers. I think that finding details to support a positive state of mood or temperament will be contingent on the individual. For example, if someone feels galvanized into a positive mood by nationalism, then one can point to specific instances in one's own national history that help to create a positive mood. A stunning piece of legislation, a historic and prominent battle, a great leader, or a moment in time when one's nation rose to a specific challenge can all be historical details that help to create a very positive mood. Another example of historical details that can help to create a postive mood could be based out of one's passions and the history within them. For example, if someone was passionate about art, examining two significant works of art in its history can help to create a positive mood. Along these lines, specific works by artists can serve as historical details that can create a positive mood in the individual. This can apply to literature and written works, as well. If someone who is passionate about a particular author can identify two specific and powerful works of that writer, then a positive mood is created when the one who appreciates such work can reflect on these endeavors. I think that the open nature of the topic creates open and divergent paths to answering it.