A few observations in no particular order:

• I would consider redesigning the class so that it presents a static method: checkDay(day, month, year). There doesn't seem to be much benefit in requiring callers to create an instance of the class, bound to a particular date. If you do this, ensure you make your constructor private so that the class cannot be instantiated.

• If you decide to stick with the current layout, then the fields day, month and year could be declared as final, since you don't adjust them at all within the class.

• A minor point, but your constructor may be more readable if you use day, month and year rather than d, m and y.

• Because your leapYear method is static, you should call it in a static way:

    public boolean checkDay() {
      if (this.month == 2 && leapYear(this.year)) { // <----- here
        Months.getMap().put(this.month, 29);
      }
  

• In general, you use this. more than is necessary. I guess it's a style issue, but generally code will only use this. when it's necessary to distinguish between local variables of the same name and class fields.

• In your Months enum, you have a field named MonthNumber, but it should be monthNumber to match normal Java style.

• You've created an enum for your month data information, but you just populate a map with the values and exclusively use the map thereafter. As a result, your enum is not really serving a purpose. If you intend to keep it private, I would consider simplifying your code and just keeping the map. If you keep the map, it could be declared final.

• It would be easier to read your leapYear method if you added some parentheses, rather than relying solely on operator precedence. You don't need the outer parentheses however:

    return (y % 4 == 0) && ((y % 100 != 0) || (y % 400 == 0));
  

A few observations in no particular order:

• I would consider redesigning the class so that it presents a static method: checkDay(day, month, year). There doesn't seem to be much benefit in requiring callers to create an instance of the class, bound to a particular date. If you do this, ensure you make your constructor private so that the class cannot be instantiated.

• If you decide to stick with the current layout, then the fields day, month and year could be declared as final, since you don't adjust them at all within the class.

• A minor point, but your constructor may be more readable if you use day, month and year rather than d, m and y.

• Because your leapYear method is static, you should call it in a static way:

    public boolean checkDay() {
      if (this.month == 2 && leapYear(this.year)) { // <----- here
        Months.getMap().put(this.month, 29);
      }
  

• In general, you use this. more than is necessary. I guess it's a style issue, but generally code will only use this. when it's necessary to distinguish between local variables of the same name and class fields.

• In your Months enum, you have a field named MonthNumber, but it should be monthNumber to match normal Java style.

• You've created an enum for your month data, however this is used solely to populate a map. The rest of your code just uses the map. As a result, I would suggest you remove the enum and just stick with a map, unless you plan to extend this class in future and use the enum for other purposes. If you stick with using an enum, the map can be declared final.

• It would be easier to read your leapYear method if you added some parentheses, rather than relying solely on operator precedence. You don't need the outer parentheses however:

    return (y % 4 == 0) && ((y % 100 != 0) || (y % 400 == 0));
  

• In your checkDay() method, I would avoid putting data into your map. Certainly you don't want to pollute your map if you plan to move to a static checkDay() method as discussed above. I would do something like this instead:

    public boolean checkDay() {
      int daysInMonth =
          month == 2 && leapYear(year) ? 29 : Months.getMap().get(month);
  
      return day <= daysInMonth;
    }
  

• (Very minor...) Typically one would call your method isLeapYear rather than simply leapYear.