It means if the value of the variable you are using in comparison is 1, 3, 5, 7, 8, 10, or 12, it would make these case statements true. As a result, the 'day' variable is assigned to 31.
If you are assigning a value to a variable, you assign only 1 value to it. If you want to compare, you could use if-else, switch, or comparison operator.
Anyway, what the "Max" value you are talking about in your post? Even though, I can understand the portion of code you gave, the portion of the code is not really completed. What are you really looking for from the code?