Keeping vrat is a process of cleansing your body and mind by fasting and namajapa and doing poojas etc. which are normally done on all holy days depending on your own convenience. If two important days fall on a given day or if holy days fall on consecutive days, it does not prevent you from keeping vrat if it is convenient to you and if not convenient you can also skip it. Vrat anushtan is not done under any compulsion from anyone or any quarters. It totally depends on your own belief and wishes and there are also no fixed modus operandi to carry out such vrats.