1. I was surprised that there was so little traffic in the morning and that I was the first one to arrive at work. When no one else had shown up by 8:30, it finally dawned on me that it was a holiday.
2. A man came to Tom’s house yesterday asking all sorts of questions about Tom’s schedule. I asked Tom why it hadn’t dawned on him that the man may have been a thief checking when Tom would be away from home.

Similar to: see the light. The expression suggests that the dawn reveals a situation. See the light simply means to understand, whereas dawn on someone suggests that one has been blind or slow to understand.