If I understand you correctly, you could simply type the date, for example 01/01 in cell A1 and the corresponding day in B1 and then highlight both by left clicking A1, hold the mouse button and drag to B1 and then release the button. In the resulting highlighted cells, hover over the very bottom right hand corner of B1 until your cursor changes to a cross and then drag down as far as you need to for each month (ie. to row 31 for January). Hope that helps.