You are doing a very responsible thing by trying to figure out the setting in preparation for your speech. However, often times the information that we want to know is not the information that the author has given to us. So, we know very little of the setting of Oedipus at Colonus. Here is what we do know about the setting.
Oedipus at Colonus was written sometime near the end of Sophocles’ life in 406–405 B.C. All of Sophocles' play were written in Athens. The time of writing though is not the same time as when the story is purported to take place. Oedipus at Colonus takes place in the mythical past in a city near Athens, called Colonus. Also it might be good to keep in mind that this work was written after Sparta defeated Athens.
If you look at the play, there are few indications of time. In addition, the play does not take place at one given time of day. In other words, the action is protracted. However, we can make one deduction of time. Polynices want to go to war against Etocles. This would mean that it is the spring season or nearing it, when military campaigns take place. Apart from this detail, there is little else. In view of this fact, you will have much flexibility to use your own imagination.