You cannot guess the exact days of when Ramadan will start, many times a calendar or website will say it's on some day, but ends up being the day before or the day after. In the end, it all depends on the sighting of the crescent the evening before the supposed day, if it is not sighted, they must wait till the next day to look for the crescent again.
Ramadan this year (in America) will begin somewhere in between the 31st of August and the 2nd of September and will last for 29-30 days, depending on the moon (should be a crescent that looks like a curved strand of hair). Regardless of which day it starts, Ramadan will be considered to have started the night before the actual day, since in Islam, Maghrib (Sunset) marks the beginning of the next day.
You can find out by calling your mosque on the 31st of August and 1st of September to find out. Calls will usually be flooded at that time, so they will record a message telling when Ramadan will be. Hope this helped :)