If somebody comes at your home without your invitation, he is called atithi. Tithi means particular time and date. But a person arrives at your home without any invitation on particular time and date, he is called atithi.
Tithi means
Lectures
Srimad-Bhagavatam Lectures
Conversations and Morning Walks
1972 Conversations and Morning Walks
Tithi means date.
Room Conversations -- April 22, 1972, Japan: Prabhupāda: Atithi means guest without any information. That is called atithi. Tithi means date. So if I go to your house, I inform you that "Such and such date I am coming there." But atithi, he does not inform you, all of a sudden comes. So you should have to receive him. That is called atithi.