| Quote #1
When the siege and the assault were ended at Troy,
The narrator of Sir Gawain begins his story after the fall of Troy - approximately 2000 years before the action begins. Why might he do this? Well, in order to get to Arthur, he has to go through a "history lite" version of the founding of Britain and the kings leading up to Arthur, which allows him to locate Arthur as part of a prestigious and long-lasting tradition of bravery, making him (and the stories surrounding him, like the narrator’s) seem very important, since they form a part of this tradition.
| Quote #2
And when Britain had been founded by this noble lord,
This passage sets Arthur (and the story) in the "age bygone" in which more wondrous events have occurred than in any other. This "bygone age" is a little like "once upon a time" - we know to expect a universe similar to ours, but one in which anything - like fairies, ogres, and giant green men - is possible.
| Quote #3
For there the festival lasted the whole fifteen days
Sir Gawain opens during the Christmas season at Arthur’s court, which, in the traditional church calendar, lasted a full fifteen days. During that time, the court gathers for the length of the whole celebration.