At that time Jesus went on the sabbath day (σαββασιν - plural) through the corn; and his disciples were an hungred, and began to pluck the ears of corn, and to eat. But when the Pharisees saw it, they said unto him, Behold, thy disciples do that which is not lawful to do upon the sabbath day(σαββατω - Singular). But he said unto them, Have ye not read what David did, when he was an hungred, and they that were with him; How he entered into the house of God, and did eat the shewbread, which was not lawful for him to eat, neither for them which were with him, but only for the priests? Or have ye not read in the law, how that on the sabbath days (σαββασιν - plural) the priests in the temple profane the sabbath (σαββατον - singular), and are blameless? But I say unto you, That in this place is one greater than the temple. But if ye had known what this meaneth, I will have mercy, and not sacrifice, ye would not have condemned the guiltless. For the Son of man is Lord even of the sabbath day (σαββατου - Singular). And when he was departed thence, he went into their synagogue: And, behold, there was a man which had his hand withered. And they asked him, saying, Is it lawful to heal on the sabbath days (σαββασιν - Plural)? that they might accuse him. And he said unto them, What man shall there be among you, that shall have one sheep, and if it fall into a pit on the sabbath day (σαββασιν - Plural), will he not lay hold on it, and lift it out? How much then is a man better than a sheep? Wherefore it is lawful to do well on the sabbath days (σαββασιν - Plural). (Mat 12:1-12)
In most cases above the plurals are translated as Sabbath days, and it makes sense that it then means on these Sabbath days "Priests profane the sabbath", "is it lawful to heal" or "to do well", but why in the Greek is the plural form used for the first occurrence (when it is a Specific Sabbath day Jesus is walking in the cornfield)?
The second last occurrence makes sense as a general rule (animals falling into pits sabbath days) but I wonder why it is translated singular in almost all Translations except the Berean and Young's Literal Translations. The first occurrence is strange though because it clearly is a specific day - the seventh.
But pray ye that your flight be not in the winter, neither on the sabbath day(plural) (Mat 24:20)
In the end of the sabbath (plural), as it began to dawn toward the first day of the week, came Mary Magdalene and the other Mary to see the sepulchre. (Mat 28:1)
And they went into Capernaum; and straightway on the sabbath day (plural) he entered into the synagogue, and taught. (Mar 1:21)
And now when the even was come, because it was the preparation, that is, the day before the sabbath (plural), (Mar 15:42)
And he came to Nazareth, where he had been brought up: and, as his custom was, he went into the synagogue on the sabbath day (plural), and stood up for to read. (Luk 4:16)
And he was teaching in one of the synagogues on the sabbath (plural). (Luk 13:10)
And they returned, and prepared spices and ointments; and rested the sabbath day (plural) according to the commandment.(Luk 23:56)
And immediately the man was made whole, and took up his bed, and walked: and on the same day was the sabbath (plural). The Jews therefore said unto him that was cured, It is the sabbath day (plural): it is not lawful for thee to carry thy bed. (Joh 5:9-10)
But when they departed from Perga, they came to Antioch in Pisidia, and went into the synagogue on the sabbath day (plural), and sat down. (Act 13:14)
And on the sabbath (plural) we went out of the city by a river side, where prayer was wont to be made; and we sat down, and spake unto the women which resorted thither. (Act 16:13)