In menstruating women, the best time to start treatment is immediately after a period. Treatment may also be started later, but it is advisable to have at least ten days in hand before the onset of the next period. Similarly, the end of a course of HCG should never be made to coincide with menstruation. If things should happen to work out that way, it is better to give the last injection three days before the expected date of the menses so that a normal diet can he resumed at onset. Alternatively, at least three injections should be given after the period, followed by the usual three days of dieting. This rule need not be observed in such patients who have reached their normal weight before the end of treatment and are already on a higher caloric diet.
Patients who require more than the minimum of 23 injections and who therefore skip one day a week in order to postpone immunity to HCG cannot have their third injections on the day before the interval. Thus if it is decided to skip Sundays, the treatment can be started on any day of the week except Thursdays. Supposing they start on Thursday, they will have their third injection on Saturday, which is also the day on which they start their 500 Calorie diet. They would then have no injection on the second day of dieting; this exposes them to an unnecessary hardship, as without the injection they will feel particularly hungry. Of course, the difficulty can be overcome by exceptionally injecting them on the first Sunday. If this day falls between the first and second or between the second and third injection, we usually prefer to give the patient the extra day of forced feeding, which the majority rapturously enjoy.
Source: Dr. Simeons Pounds and Inches