First make sure you are supervised by a medical doctor. Gaining weight is like loosing weight. We need to know if we are healthy first before we go doing anything.
See your doctor an rule out any eating disorder, hormonal issues or stress.
When you say you want to gain weight by a certain date, it indicates an urgency verses a positive weight gain. See a doctor.
Go to your health food store and get a good, weight gain, protein drink, this is the safest and most effective way to gain weight.
Try eating more fatty foods like bacon, roasts, or ribs. You incorporate bacon into almost any meal. Also instead of getting lean beef just get normal.