Why should the tribulation flight not take place on the Sabbath day?
Pray that the tribulation flight not occur on the Sabbath day because, under the law, Jews were limited in how far they could walk on the Sabbath. That legal restriction would hinder or prevent their escape, so Jesus warned them to pray it would not happen then.
Les explains that verse 20 indicates the Jew will be back under the law. According to that law, they were limited as to how far they could walk on the Sabbath day, so Jesus said they should pray the flight not happen on the Sabbath; otherwise the law would limit them from being able to escape. He also notes they were to pray it not be in winter, when cold, snow, rain, and ice can occur in Israel. The flight is a sudden, providential move to a distant place, likely southeast of Jerusalem, possibly to the area of Petra, and it resembles the Exodus out of Egypt. This occurs in the middle of the tribulation, the last three and a half years, which will be unprecedented, though those days will be shortened so survivors remain.
But pray ye that your flight be not in the winter, neither on the sabbath day: