Question: At the beginning of Ramadaan in the past year, we kept waiting for hearing anything about the start of the month, but we could not. We slept without having intention to observe Sawm (Fast). In the morning, we learned that Ramadaan had started on that day and so we observed Sawm on that day (first of Ramadaan). What is the ruling on our Sawm? Have we to make up for that day?
Answer: If the case is as you mentioned in your question, it is obligatory upon you to make up for that day because you did not have the intention from the previous night... read more here.