December has 31 days in the Gregorian calendar, as that is the calendar that we now use.
Not in the Gregorian calendar.
The shortest "month" of the Gregorian calendar has 28 days.
No months in the Gregorian calendar have 80 days.
The eighth month of a year of the Gregorian calendar, August, has 31 days.
April has 30 days in the modern Gregorian calendar.
July is the seventh month of the year in the Gregorian Calendar and one of seven Gregorian months with the length of 31 days.
In the Gregorian calendar, each month does not - so the question is misguided.
The month in which the Gregorian Calendar was adopted, because when it was proposed in 1582, there was a difference of 10 days between it and the earlier Julian calendar, and that difference had grown to 13 days by the time that the last countries adhering to the Julian Calendar (Greece and Russia) switched over to the Gregorian Calendar. Because of the "loss" of between 10 and 13 days, the month in which a particular country adopted the Gregorian Calendar might not have had a full moon.
The Islamic calendar is about 11 days shorter than the Gregorian calendar, so Ramadan, or any Islamic month for that matter, will not always be in the same month in the Gregorian calendar. According to the Wikipedia, in 2011 it is from August 1 to August 30.
That depends what calendar you are talking about. Both the Jewish and the Islamic (or Hijri) calendars have calendar months that coincide with the lunar cycle - that is, with the lunar month. But you are probably more familiar with the Gregorian Calendar - in this case, the months are slightly longer than the lunar months. A lunar cycle is about 29.5 days; the months in the Gregorian calendar have almost 30.5 days on average.
The Gregorian Calendar is an almost identical improvement on the Julian Calendar. The names of the months and the number of days per month are the same. The only difference is that the Gregorian Calendar has three fewer leap year days out of every 400 years. The Julian Calendar averages 365.25 days per year, and the Gregorian Calendar averages 365.2425 days per year. It doesn't seem like much, but after using the Julian calendar for 1 1/2 millennia the accumulated error totaled about 10 days.