When and how is the intention for fasting made?

Question:
Does a person make the intention for fasting at night?

Answer:
It is obligatory for him to have his intention for each day; the day is independent. This is the correct view.
Q: What about getting up for the pre-dawn meal?
The Shaykh: If he gets up for the pre-dawn meal, he has intended to fast, so he does not need to [make the intention again].
Q: Is it sufficient to make one intention at the beginning of the month?
The Shaykh: It is not sufficient. The correct view is that an intention is necessary for every day. If he fasts while being heedless and did not intend anything, it does not suffice for him. It must be independent for every day. If he ate in it, he broke his fast; if he had intercourse in it, he broke his fast. Every day has its own independence.
Q: And what if he forgot?
The Shaykh: As long as he intended in the night that he would fast it, that is sufficient. As long as his intention is that he will fast it tomorrow, praise be to Allah, even if he did not remember at the time of the pre-dawn meal.