The last 3 times you went to the dentist for your 6-month checkup, it rained as you drove to her office. What is the probability that it will rain while you are driving to today's dental appointment?
Based on the information given, this is an impossible question to answer.
However, the point of this is to consider independent events. In this case, going to the dentist is likely to be independent of the weather. In addition, given that the appointments are 6 months apart, the weather at each event is likely to be independent of each other.
If these two statements are true, then it means both that a) the fact that you are going to the dentist does not affect the chances of rain and b) the fact that it has rained in the past does not affect the chances of rain.
In conclusion, the probability that it rains today is based solely on the overall probability of rain for that day. If you don't know the date, then this is the proportion of days that it rains locally. If you do know the date, then you may need to consult a meteorologist to determine the chances of rain (although who trusts the weatherman anyway?!?).