"Wherein our Saviour's birth is celebrated..."
See in text (Act I - Scene I)
The birth of the "Savior" Jesus Christ refers to the celebration of Christmas, establishing that it is currently Winter in Denmark. Able-bodied men didn't have to tend to the fields during Winter and were instead available to fight in the upcoming war. This line also indicates that these characters live in a world of superstitions and that, for them, myths and legends are often as important as reality.
"simple and unschool'd..."
See in text (Act I - Scene II)
As a young scholar, being called "simple and unschool'd" would've been a particular blow to Hamlet. He may be young and inexperienced, but he does seem to pride himself on being thoughtful and intelligent. Combined with Claudius' criticisms of his piety and masculinity, Hamlet's continued grief has been thoroughly admonished as ridiculous and unnatural. Keep in mind that this is not a private conversation either. Claudius has made all of these remarks in front of the attendant lords and ladies of the realm, undermining Hamlet on both a personal and social level.