It already started, but many stores have xmas boards, with a gift for day along December. If we can make a list, I guess it will help many people... Some confirmed. With help, we can keep updating it.
Update: my "condensed" personal list. I'm removing what I don't care about, like furniture, kids', furry and fantasy stores, keeping clothes, makeup, jewels, body parts, etc. Most of are free to join, some don't even require joining. Sadly, some will give only the gift of the day (older can be obtained by low prices, or are lost).
To top it off, some have excellent lucky chairs, boards, raffles and random gifts. All tested, more tomorrow (hoooarding :p).
7 Deadly Skins (sorry, no link at the moment)
Abranimations http://maps.secondlife.com/secondlife/Animations/100/159/24
ALB Fashion (you can get 7 gifts in a row, I'll return only in Dec 9) http://maps.secondlife.com/secondlife/Millais/218/15/21
Amazing Creations (3 trees for men, women and mix) http://maps.secondlife.com/secondlife/Ocean Retreat/174/223/24
Annex Mainstore (1L to get, paid back; older gifts 25L. Special gift Dec 25 available ONLY that day) https://maps.secondlife.com/secondlife/Paper Dinosaurs/146/127/21
AppleTini (nail appliers) http://maps.secondlife.com/secondlife/Florida/147/234/702
Applique Chic http://maps.secondlife.com/secondlife/Woodland Park North/85/233/21
Bella Bee (gifts under the tree) http://maps.secondlife.com/secondlife/Florianopolis/40/213/1503
Caledon Oxbridge https://maps.secondlife.com/secondlife/Caledon Oxbridge/93/194/2
Carrie Snowpaws (good lucky letters by the way...) http://maps.secondlife.com/secondlife/Distant Lights/228/65/24
Christmas Delight (some free stuff on 2nd floor, too) http://maps.secondlife.com/secondlife/Jewellery/197/75/38
Diva Couture http://maps.secondlife.com/secondlife/Oppenheim/166/229/23
Donnie Dollies http://maps.secondlife.com/secondlife/New Vice City1/68/186/21
Dulce Secrets (skins, makeup) http://maps.secondlife.com/secondlife/Comhar/129/56/23
Ecletic Stars (geeky clothes) http://maps.secondlife.com/secondlife/Sunspot/112/58/72
Entice http://maps.secondlife.com/secondlife/Flawless/77/209/28
EscalateD (hair; special gift in Dec 25; 250L to join, now free, great lucky letters, too) https://maps.secondlife.com/secondlife/Chrysanthemum Island/172/37/21
Firelight (hair) http://maps.secondlife.com/secondlife/Flawless/43/69/28
Formanails http://maps.secondlife.com/secondlife/Formanails/121/165/31
Go Mainstore (gifts next to fireplace) http://maps.secondlife.com/secondlife/Sugar Plum/150/94/24
Heavenly Creations (fashion, join free required, 2 trees, 1 for members-only) http://maps.secondlife.com/secondlife/Foxville/36/120/24
Kingbal Stores http://maps.secondlife.com/secondlife/Love Dolls Island/101/98/22
La Bella Boutique http://maps.secondlife.com/secondlife/Everstream/137/173/3002
PoD (adult, BDSM) http://maps.secondlife.com/secondlife/PoD BDSM Resort/57/231/59 ReACT https://maps.secondlife.com/secondlife/Akaesha/209/38/23
Sexy Princess - 2 panels http://maps.secondlife.com/secondlife/The Crossroads Event/55/33/2503
Shoediction (2 trees, 1 for member-only) and Emerald Couture (special gift in Dec 25) http://maps.secondlife.com/secondlife/Tamna/60/68/2529
SLInfo.de (boxes under tree, click until find the right one) http://maps.secondlife.com/secondlife/Junapel/152/220/2402
Wellmade - idem, boxes inside http://maps.secondlife.com/secondlife/Hutter/102/117/23
Seems to be not so good quality, maybe I'm harsh...
A&A Hair http://maps.secondlife.com/secondlife/Wonderland Beach/127/126/23
Alice http://maps.secondlife.com/secondlife/Panzerotto/20/239/1999
Specific date gifts or not started yet
AMA (adult theme poses, Dec 13) https://maps.secondlife.com/secondlife/Moonspell/95/188/993
Altamura (special gift - probably a head? - in Dec 24 and 25) https://maps.secondlife.com/secondlife/Nolan/197/17/31
Baby Monkey (no join required, Dec 13) http://maps.secondlife.com/secondlife/Keraxic/53/183/26
Cleauf de Peau (skins, 100L, lots of gifts in Dec 23 ONLY) http://maps.secondlife.com/secondlife/Clef de Peau/146/157/22
Essenz Mainstore (shoes, 350L but free to join between Dec 7-10, gifts from 13-24) http://maps.secondlife.com/secondlife/Fruitatious/47/200/24
Paid join required
Allura 100L http://maps.secondlife.com/secondlife/Deneb/81/162/2018
Celestinas Wedding 150L http://maps.secondlife.com/secondlife/DreamsLand/210/152/3002
Chop Zuey (jewelry, 350L, starts Dec 12) http://maps.secondlife.com/secondlife/Chop Zuey/52/143/26