Upper Saxony
It was particularly to distinguish the lands from 'Lower Saxony', a concept which arose later in popular usage (though never enforced) as a term for the original Saxon lands in north and west Germany (where Low German dialects had spread), in what is now the state of Lower Saxony, as well as the adjacent Westphalian region, Holstein and the western part of today's Saxony-Anhalt north of the Harz range (Eastphalia).
Saxony
The Electoral Saxony, a predecessor of today's Saxony, was the original birthplace of the Reformation. The elector was Lutheran starting in 1525. The Lutheran church was organized through the late 1510s and the early 1520s. It was officially established in 1527 by John the Steadfast. Although some of the sites associated with Martin Luther also lie in the current state of Saxony-Anhalt (including Wittenberg, Eisleben and Mansfeld), today's Saxony is usually viewed as the formal successor to what used to be Luther's country back in the 16th century (i.e. the Electoral Saxony).