Hi, Josh —
Friday is the 6th day of the week. In most of the Gospels, it records that Jesus was taken down from the Cross before the Sabbath (or before the 7th day) which is Saturday. That's what Sabbath means — the 7th day but in the Hebrew Tradition, a day starts at sun down. So when the sun sets on what we call Friday, the Jews begin to celebrate the Sabbath or the start of the 7th day.
We understand this from Genesis chapter 1 when we see the days called off. Thus was the evening and the morning of the 1st day. (Genesis 1:3-5) They always started from the evening, so it was from sun down to sun down, not from midnight to midnight as we do it. In other cultures they did it from sun rise to sun rise.
As far as the Jehovah witnesses are concerned, first of all you're dealing with an extreme form of Fundamentalism. Secondly, you're dealing with a cult. They routinely translate texts from the Hebrew and the Greek so, in essence, you really need a ton of training to deal with them.
I would suggest that you consider learning your Catholic faith at a deeper level first, rather than engaging or debating cults or other denominations who are trying to convert you.
Hope this helps,