It is often noted Jesus in some form repeated nine of the Ten Commandments; omitting the one to keep the Sabbath. Thus one may reason: by failing to state the necessity, continuing to keep the Sabbath is no longer a requirement. I find this line of reasoning overly simplistic, if not naïve. For example, if Jesus had failed to restate the commands not to kill, or steal, or bear false witness, or commit adultery, would these behaviors now be understood as no longer prohibited? Does any Christian really believe "liberty" from the Law means behavior is no longer subject to the Law?
Jesus required those who believe to be followers. Jesus observed the Sabbath and logically His followers should do likewise; not according to manmade rules, by according to Scripture. Jesus specifically addresses this issue in John:
19 Has not Moses given you the law? Yet none of you keeps the law. Why do you seek to kill me?” 20 The crowd answered, “You have a demon! Who is seeking to kill you?” 21 Jesus answered them, “I did one work, and you all marvel at it. 22 Moses gave you circumcision (not that it is from Moses, but from the fathers), and you circumcise a man on the Sabbath. 23 If on the Sabbath a man receives circumcision, so that the law of Moses may not be broken, are you angry with me because on the Sabbath I made a man's whole body well? 24 Do not judge by appearances, but judge with right judgment.” (John 7) [ESV]
If one is commanded to follow, it is unnecessary to restate a previous positive instruction. Hence the overly simplistic argument of making significant Jesus' failure to restate the command. If there is an issue, it is Jesus' failure to completely recast the command in the negative: do not stop keeping the Sabbath.
For followers it is necessary to remember, restating prohibited behavior is different from restating required behavior. For example, since Jesus did not marry on earth, the command not to commit adultery must be restated. This type of restatement is not a law; it is required because Jesus cannot provide His followers with the positive example of His own actions. So restating any prohibition is simply an affirmation to continue to follow Jesus' example.
Keeping the Sabbath is something followers should do because that is what Jesus did, and the issue is how, not if Jesus kept the Sabbath.
The passage needs to be considered in context:
15 “So when you see the abomination of desolation spoken of by the prophet Daniel, standing in the holy place (let the reader understand), 16 then let those who are in Judea flee to the mountains. 17 Let the one who is on the housetop not go down to take what is in his house, 18 and let the one who is in the field not turn back to take his cloak. 19 And alas for women who are pregnant and for those who are nursing infants in those days! 20 Pray that your flight may not be in winter or on a Sabbath. (Matthew 24)
Jesus commands a certain group to take a certain action:
...then let those who are in Judea flee to the mountains...pray that your flight may not be in winter or on a Sabbath.
Whether Christians should observe the Sabbath is a good question, but this particular passage cannot be taken as anything more than instruction for those who are in Judea to flee to the mountains. That is to say, just as no one sees this as a call for all people to go to the mountains; this should be approached as instruction to take flight for those specifically in Judea.
First, Jesus linked winter and the Sabbath. Obviously man has no control over "observing" winter. Then to pray the flight does not occur in winter is simply a practical consideration:
...let those who are in Judea flee to the mountains...pray that your flight may not be in winter...
Not only is travel in winter more difficult, fleeing to the mountains (the positive command) makes the destination even more extreme. The proper interpretation is to pray the required travel will not occur during a season in which travel is the most difficult and the destination will be the most inhospitable.
The Sabbath is marked by human activity or inactivity. In Judea, or any place with a large Jewish population, the Sabbath will be identified by a cessation of normal work and commerce. A trip which must be made quickly will be more problematic on a Sabbath. Not as a result of breaking a prohibition on how far one may travel on the Sabbath (something not specifically addressed in the Law); rather, it recognizes a trip on the Sabbath will be more difficult because food and clothing for the journey will be difficult, or even impossible to obtain:
then let those who are in Judea flee to the mountains. Let the one who is on the housetop not go down to take what is in his house, and let the one who is in the field not turn back to take his cloak...pray that your flight may not be in winter or on a Sabbath
The one at home is not to go inside to prepare for a journey. Arguably, in winter they would be wearing a coat, but not have food ready for a trip. On the other hand, one in the field would likely not be in the winter and not wearing a coat (which would be needed upon arriving in the mountains).
In Matthew 24 Jesus is addressing the practical reality of those in Judea "dropping everything" and fleeing to the mountains for safety. In Judea, businesses will be closed on the Sabbath. A trip in winter or when food and clothing cannot be purchased along the way will be more difficult.
Therefore it is reasonable to conclude Jesus expects the Sabbath will be observed in Judea and that would make unplanned travel on a Sabbath more difficult. However, those making the journey will not be violating the Sabbath: they will following the instructions of Jesus.
Since this particular instruction is specific to hastened travel in Judea to the mountains, it neither adds nor subtracts from Jesus' expectations for His followers to observe the Sabbath.