Historical Context in Ode on a Grecian Urn

Text of the Poem 2
"O mysterious priest..."   (Text of the Poem)

Most likely the priest of Hera, queen of the gods, or Hymen, god of marriages. In ancient Greece, both these gods traditionally presided over weddings, requiring that a heifer (or cow) be sacrificed in order to bless the marriage.

"Cold Pastoral..."   (Text of the Poem)

In ancient Greece, the pastoral referred to the ideal state of nature. In poetry, the pastoral is a type of poetry that glorifies that ideal nature. It's described here as being "cold" because, though not technically alive (or warm), it still lives forever.