On watch in the Forest of Dean, Harry sees a silvery doe and follows it to discover Gryffindor’s sword in a frozen lake; when he tries to get it, Ron appears, saves Harry’s life when the locket tries to strangle him it the water, and uses the sword to destroy the locket Horcrux. Harry later discovers that the doe was Snape’s patronus; he had placed the sword at the bottom of the lake, and was leading Harry to it.
Harry saved Ron from a lake during the second task of the Triwizard Tournament (GF26). -BB
In Arthurian legend, some accounts say that King Arthur received the sword Excalibur from the Lady of the Lake. (source: Wikipedia) -BB
Christian symbolism in the chapter "The Silver Doe" has been the subject of much conjecture, with many equating the silver doe with Christ and concluding that Harry's submergence represents baptism. (sources: How Harry Cast His Spell by John Granger, The Gospel According to Harry Potter by Connie Neal) -BB
Snape sent the Silver Doe on December 26th, the Feast Day of St. Stephen a Bohemian king, mentioned in the famous song "Good King Wenceslas." The snowy night and the fact that Harry followed the doe are much like the servant following in the footsteps of the king in the old Christmas carol. St. Stephen's Day is associated in various countries with sleigh rides and the holly tree, which is Harry's wand wood.
December 26th is also Boxing Day in the UK, a time when: "it was a custom for tradespeople to collect "Christmas boxes" of money or presents on the first weekday after Christmas as thanks for good service throughout the year....This custom is linked to an older British tradition: since they would have to wait on their masters on Christmas Day, the servants of the wealthy were allowed the next day to visit their families. The employers would give each servant a box to take home containing gifts, bonuses and sometimes leftover food" (Wikipedia: Boxing Day).
From the Web
Official website on the Forest of Dean region: http://www.wyedeantourism.co.uk/
St. Stephen's Day on Wikipedia
Good King Wenceslas on Wikipedia
Boxing Day on Wikipedia