What is the meaning of Many happy returns of the day and what is the context it is used in and usually by whom?

It helps to take that in a slightly different order: There isn't really a particular person who says it. It's normally said to a person celebrating a birthday or other anniversary. It's a way of expressing the hope that they will have many more birthdays (or anniversaries), and that those will be happy occasions... in other Words, "Happy Birthday, and I hope you have many more."