I need to be able to click on a cell to bold/unbold the number in the cell

I have a monthly vehicle report that I am revamping and I need to build some features into it to increase user friendlieness. I need to be able to click on a cell that has a day of the month in it, to bold or unbold and count/uncount the number of bold days. For example the days of the month will be in cells B6:I9 and the tally box at J7.