Friday from sunset to Saturday sunset is the Sabbath why do we see Sunday as the Sabbath?

Answer 1

Due to the Jewish wars in circa 132-135 AD, a wave of anti-Semitism spread through the Roman Empire. This caused many new Christians - many of whom were Jewish - to separate themselves from anything 'Jewish.' This included the Christian and Jewish Sabbath which Jesus and the Apostles kept faithfully. At the same time, pagan (mostly ascetic and gnostic) beliefs started to exert influence on the new Christians (see Acts 20:29-30).

In this environment, along comes Emperor Constantine I, having defeated his only rival Maxentius in 312 AD using 'Christian' symbols on the shields of his soldiers. Though a pagan 'sun worshipper' or Sol Invictus (dies solis), he now began using 'syncretism' to merge or consolidate his power over the pagan Romans and new Christians. One of his first changes was to adopt Sunday in-lieu of Saturday as the only day of worship. Anything Jewish he despised. A bit later in the 4th Century, another Roman Empire made Sunday worship mandatory upon death if caught worshipping on the 'Jewish Sabbath' (Note: It is God's Sabbath established in Creation week long before anyone called a Jew). The Roman Church grew in power and land areas - today the Roman Catholic Church is the largest with 2.1 billion adherents. And the 'daughter' churches in Orthodoxy and Protestantism simply kept this wrong practice. There was no divine authority to change God's Sabbath - regardless how one wrongly interprets Matthew 16:19 and the future perfect passive tense it uses in its construction. Remember what the Scripture says very clearly for both physical and spiritual Israel (Church today):

Exodus 31:13New American Standard Bible (NASB) 13 "But as for you, speak to the sons of Israel, saying, 'You shall surely observe My sabbaths; for this is a sign between Me and you throughout your generations, that you may know that I am the Lord who sanctifies you.