Google Sheets – How do I move a value and block it?

I'm looking for a way to take a value from one cell and move it to another, and the cell it moves to does not change until a replacement value is entered. It seems simple, but the value for which I am changing the changes is based on the date, and there are some days when there is no value but I want to keep the value of the previous days. Does anyone know how to do it or if it is possible?