Five free loot boxes are not listed in the event details Blizzard is shared back when the event is started, but according to an in-game pop-up that appears when the players log in, the loot box appears to be a "thank you" gift from Blizzard.  HERE IS MERCY SKIN OF MY DREAMS with complimentary 5 winter loot boxes. #Notwatchmercy #lootbox – Ammy Star (CorgiBeans) (@AmmyStarForgi) December 24, 2018
Overwatch JUST gave me 5 winter loot box for Christmas. Of course, 4 of them are all double things, BUT this one is a very good box. pic.twitter.com/M9Payl3J79
– Steven Shaggy Shan (@Shaggyshan) December 24, 2018
"Heroes, it really was an event and a wonderful year, and wanted we take a moment to thank you for being part of it, "said the in-game message. "We can not do this without the intense support, love, and enthusiasm you have shown for Overwatch . From the entire Blizzard team, we ask you and you the best and hope for spending an exciting new year to you! "
No end date or time is provided for promotion, so do not know how long the players are to log into the game and collect their free loot box. The Winter Wonderland event is scheduled to end on January 2 though, so logging in before that until more information is known will be a safe bet to ensure the loot boxes are retrieved.
Like other themed loot boxes issued or sold by Blizzard, Overwatch's Winter Loot Boxes will give players all kinds of new cosmetics if they are lucky enough to score items from randomized rewards loot.
"New seasonal items can be found in the Winter Loot Boxes full of new skins, emotes, highlights, player icons, sprays, voice lines, and more," Blizzard said in boxes of robbery. "You can also buy items you did not get from last year's event with discount credits. But do not wait too long-Loot Boxes will be gone faster than holiday desserts."  Overwatch's Winter Wonderland event lives today until January 2 in full detail of the event located here.