Matthew 17:1

1 And after six days Jesus taketh Peter, James, and John his brother, and bringeth them up into an high mountain apart,

Mark 9:2

2 And after six days Jesus taketh with him Peter, and James, and John, and leadeth them up into an high mountain apart by themselves: and he was transfigured before them.

Luke 9:28

28 And it came to pass about an eight days after these sayings, he took Peter and John and James, and went up into a mountain to pray.

Above, we can see that Matthew and Mark both say that Christ, Peter, James, and John, all went up the mountain 6 days after the previous scene in which Christ was speaking. However, Luke says it was 8 days, not 6.

There are a few ways we can resolve this. Firstly, Luke says "about" 8 days, which is an approximation. That, by itself, is a fine resolution, as in the context of the story, the precise amount of days isn't a crucial detail. The only meaningful reason for including the days at all is because Christ had just stated that certain of the disciples would "see the Kingdom of God come with power" (Matthew 16:28, Mark 9:1, Luke 9:27), in their lifetimes. All three accounts jump immediately to the Mount of Transfiguration. So the point being made by including the days is that not only did the prophecy come true, it came true immediately.

We can also recognize that different people can count days differently. It could be that Matthew and Mark only counted the 6 days between the day of the prophecy, and the day of the event, whereas Luke included both the day of the prophecy and the day of the event in his tally.