Either is fine if the sentence is short. For example, you could say either "Yesterday I went to the store," or "I went to the store yesterday." Similarly, you could say either "Tomorrow, I will go to the store," or "I will go to the store tomorrow." If you are describing several actions that you took yesterday, it is a little clearer to put "yesterday" at the beginning of the sentence. For example: "Yesterday, I went to the city, saw my friend, and bought a couple books." Similarly, it might be a little better to put "tomorrow" at the beginning of the following sentence: "Tomorrow, I will go to the city, see my friend, and buy a couple books." That way, it is clear that each of these actions in the sentence will take place tomorrow.