I am 24 years old boy who is under-weight for my age. I did my BMI and it showed that I am underweight for my age. The fast here is about 19 hours and when I am fasting I feel lazy all day and not able to do any kind of work. I am just lying down in bed all day and only getting up for the fard prayers. I was just wondering that if I don't keep the fast in this month and keep them all in December when the days are shorter, will that be acceptable in Islam?
The Shari'ah has given us some concessions in fasts. If we are sick we can leave the fasts to be made up later when we are healthy. It is upon you and the doctors to decide whether your condition allows fasting or not. If you believe that your health condition will deteriorate because of fasting you can leave it and make up the left over fasts later when you are able to fast.