3 hours before bedtime. It is much easier to fast while sleeping vs when awake. If you're running a 500 calorie deficit from your TDEE and every night done eating by let's say 7pm and you're sleeping 7 hours minimum (this won't work if you're neglecting sleep. This is vital for success), even with no exercise, you'll release weight.

Now if you add a nutrient dense diet to this along with mild exercise (a two mile walk each day) you will release weight consistently over time and not starve yourself silly.

It won't be as fast in results, however 20lbs in a month isn't impossible, however even if released half of that over a year that's 120 lbs and without you rubber banding back up because your trying to starve yourself