Weigh yourself max. 1x per week at the same time of day and under the same conditions, preferably in the morning and on an empty stomach.
Short-term fluctuations in body weight are due to the water balance. It is important that the trend is downwards.
Also take your body measurements 1-2 times a month, so you can keep a closer watch on your progress.
Read more about how to best measure your fasting success here.