You would benefit from some looping action most definatly in this application. I'll start at the top with a minor suggestion:
Instead of checking for valid days and allowing the user to enter a number again, you should just loop until valid input is given. Like so:
As for getting each day displayed you have several problems as well as several solutions to each. For starters, you're going to need to get the info for each day, not just the first one as you do here. In order to do this you're going to need an array of data. You can either set a maximum static value (like 100 or something) and have a static array length, like: long double pay; or you can allocate it dynamically when "days" is defined. It would appear that the first option is more on the level of what you're doing. You'll need to loop like so:
cout << "Please input the number of day for calculation, do NOT enter a value which less than '1'!\n";
cin >> days;
cout << "\n";
}while(days < 1);
After that you'd make another loop to display what you've got.
long double pay;
for(int i = 0; i < days; i ++)
cout << "Please input the pay for the first day (in penny)\n";
cin >> penny;
pay[i] = penny * 0.01;
cout << endl;
Try this out and post here again if you need more help or have further questions.