Talmud for Pesachim 140:3
דמית נשיא דמית נשיא אימת אילימא דמית בשלשה עשר סכין למה לי דמטבליה ואלא דמית בארבעה עשר מאי שנא סכין דמטביל ומאי שנא קופיץ דלא מטביל לה
the passover-offering unto the Lord thy God, of the flock and of the herd'. Now, does then the Passover offering come from the herd: surely the Passover offering comes only from lambs or from goats? But [it means] the left-over of the paschal offering is to be [utilized] for something which comes from the flock and from the herd. Now according to the Rabbis, what is the reason that it [the hagigah] does not override the Sabbath, seeing that it is certainly a public sacrifice? - Said R. Illa'a on the authority of R. Judah b. Safra: Scripture saith, And ye shall keep it a feast [hag] unto the Lord seven days in the year. 'Seven!' but there were eight? Hence from here [we learn that] the hagigah does not override the Sabbath. When Rabin came, he said: I stated before my teachers, Sometimes you can only find six, e.g., if the first day of the Feast [of Tabernacles] fell on the Sabbath? - Said Abaye: That Abin the childless should say such a thing! Eight is altogether impossible, [while] seven are found in most years. <br>