(Is it a set?) Set of all months having more than 28 days. 
Set is a well defined collection of distinct objects.

Is the following is a set?
Set of all months having more than 28 days.
I'm confused here. Because on one hand I think that it is well defined because from person to person its meaning is not changed. On the other hand I think that it is not well defined because if we consider a leap year then February is included else not. Note that the year is not specified, I'm you cannot surely say that February is included or not.

Set of eleven best cricketers of the world.
This is not a set because the criteria for best cricketer changes from person to person.

So the set of all months having more than 28 days. Is it really a set?

Given that you say that the "Set of eleven best cricketers of the world [...] is not a set because the criteria for best cricketer changes from person to person," then wouldn't the "Set of all months having more than 28 days" also be not a set because the number of days of the month changes from year to year?
