The differences between Mark 9.2 and Luke 9.28 regarding the number of days may seem puzzling.
In the Gospel of Mark 9.2, it is mentioned that the Transfiguration takes place "six days later," whereas in the Gospel of Luke 9.28, it is written "about eight days after."
The authors of the Gospels sometimes used different literary styles and counting methods. For example, Mark may have counted the exact days, whereas Luke may have included the partial days before and after the events described. Thus, these two accounts are not necessarily contradictory: Mark perhaps refers to the days between the two specific events, while Luke includes the days of Jesus’ teaching to his disciples as well as the day of the Transfiguration.
The differences observed in the Gospel accounts often result simply from a lack of contextual information. The authors of our Gospels had to respect the length constraints of their documents, which limited the amount of detail they could provide.