Pesachim 61 - Who comes to the Seder?

People used to eat the Passover offering in groups. These groups had to be designated beforehand. When one was slaughtering the offering, he needed to have the invitees in mind. Also, the people in a group needed to have other qualifications: they had to be healthy enough to eat a piece of meat as big as an olive, they had to be circumcised, and be ritually purified by going to a mikveh. 

If one slaughtered the sacrifice for people who could not eat it, being sick or too old, or for people otherwise disqualified, the sacrifice itself became invalid. However, if he had both categories of people in mind, qualified and disqualified, the sacrifice remained valid. 

If he slaughtered the sacrifice too early, before midday, it would become invalid because it had to be "in the afternoon." However, if he slaughtered it before the afternoon tamid (daily) offering, it remained valid - provided that someone stirred the blood so that it did not congeal until the time come to throw it. In fact, even if he threw that blood before this time, the sacrifice would remain valid.

Art: Old Man in a Cap, by Rembrandt