Q: If we cannot see the moon on the 29th night of Shaban, is it good to fast the following day or is that disliked?
A: If the sky is clear and the horizon is unobstructed on the 30th night (the evening of the 29th) of Shaban and the moon is not seen, it is disliked to fast the following day unless it coincides with a personal habit of fasting. This is known as the day of doubt.
If the sky is covered or the horizon is obstructed it is obligatory to fast with the intention of Ramadan as a precautionary measure. You should pray tarawih and the remaining rulings apply, except a divorce or manumission of a bond servant attached with the entry of Ramadan, so long as it is not proven to be the 30th day of Shaban.
If it is later revealed that it was actually Ramadan, that day will count and not need to be made up.
Source: Hidayat al-Raghib