Deciding when to to Punta Cana will make a big impact on your stay.
Weather is obviously the most important factor. Although it is mostly sunny and dry it is worth to note that during hurricane seasons once in a while hurricane can hit the island. Mostly it means a lot of rain in a short period of time and there is no reason to worry, but it is good not to be surprised.
Also it is good to know when big holidays are and when you can expect thousands of college kids party around Punta Cana.