Yoon Kwang-yong, 31, began his diet on October 16 in an attempt to re-enact Super Size Me... But he stopped on the 24th day after doctors said his health was deteriorating so rapidly that his life could be in danger. During the 24 days, Mr Yoon put on 3.4 kilograms despite daily exercise he said he took in order to ensure that the experiment was fair. We use 'on' and 'during' to describe what happens in a specified time. If you wish to mention the particular day when something happens, you use 'on'. It can be a specified day (e.g. on Monday), a day referred to by numbers (e.g. on November 16, 2004, on the 16th), or a day referred to by a special term (e.g. on my birthday, on our wedding anniversary). However, we use 'during' when we mention a period of time which is usually more than one day (e.g. during the winter, during November, during 2004). Note that it can also be a particular part of the day (e.g. during the morning).