The probability of an event in any situation is defined as the ratio of the number of outcomes favorable to the event to the total number of outcomes.
For example, take the case of determining the probability of a particular day in a week being a Monday. When a day is chosen there are a maximum of 7 possible outcomes. Of these only 1 is favorable to the day being a Monday. This gives the probability of any random day of a week being a Monday as 1/7. Extending this, consider the probability of a day chosen in a month being a Monday. Here, the number of days favorable to the event is the number of Mondays in the month which can be either 4 or 5. The total number of days in the month can be 28, 29, 30 or 31. The actual values here would determine the probability of a random day chosen in the month being a Monday.