public boolean isSummer(String month) VERSION 2

The String method equals() is case-sensitive
i.e. capital and lowercase letters will not match.

There is a second version named equalsIgnoreCase() that
is case-INSENSITIVE.

If you use equalsIgnoreCase(), all of the following will match:
"June", "JUNE", "june" and even "JUne" and "juNE".

public boolean isSummer(String month) {
  boolean summer = false;
  if (month.equalsIgnoreCase("june")) {
    summer = true;
  }
  if (month.equalsIgnoreCase("JULY")) {
    summer = true;
  }
  if (month.equalsIgnoreCase("august")) {
    summer = true;
  }
  return summer;
}

Note that another way to code this, if you want to use equals()
is to use toLowerCase()

public boolean isSummer(String month) {
  boolean summer = false;
  month = month.toLowerCase();
  if (month.equals("june")) {
    summer = true;
  }
  if (month.equals("july")) {
    summer = true;
  }
  if (month.equals("august")) {
    summer = true;
  }
  return summer;
}