The first time entering ketosis can be difficult. Especially if you have reduced carbs but not increased fats to make up for the loss of calories. During the 1-4 days it takes to enter ketosis, calories should be maintained to minimize hunger. However, as glycogen stores are used up, the body will have an increased appetite for carbs, making it difficult to continue.
To minimize this effect, maintain calories until you have transitioned to ketosis. Once in ketosis, the body will lose the cravings for carbs because it has a ready supply of sugar. At this point, you can taper back on calories without the hunger problems typical of calorie-reduced diets. Reducing calories before that point is not sustainable. Reducing calories during ketosis can be maintained until such time as you increase carb intake and leave ketosis.