Many of us are actually children of Israel by flesh not knowing it (split into the world after Babylonian captivity and other events). But every true Jesus believer is child of God (same as Israeli natives are) by adoption (grafted into good olive tree, see Romans 11:24). But you're right celebrating Sabbath is not an issue for Christians. If you are led to celebrate it to glorify Jesus or Father then do it, if you are not then don't burden yourself, you are not judged and those celebrating be not judged as well.