попробуйте с этим кодом,
SimpleDateFormat day = new SimpleDateFormat("dd");
Calendar calendar = Calendar.getInstance();
List<String> allDays = new ArrayList<>();
for(int i = 0; i < calendar.getActualMaximum(Calendar.DAY_OF_MONTH); i++){
allDays.add(i, String.valueOf(i+1));
}
![this will be your allDays list.](https://i.stack.imgur.com/HcAZp.png)