I've had a little trouble trying to decipher the calendar over at pensil's Monster Hunter site. Apparently, their Market Special days are different than ours (their servers must be synced differently as far as the day special). However, one thing that is apparent, is it seems the specials appear every other day. From what I understand, there are four different possible special days. Three of the days have items that are not usually sold (there are 3 vendors besides the Combination vendor, so I'm guessing these days relate to a specific vendor). The fourth day is half-price day. On this day, you guessed it, almost all the items sold in the market are only half their normal price.
Anyhow, I'm going to continually edit in this post as I observe the different specials and hopefully there will be some sort of trend. Also, I'd like to list the unique items that are not usually sold unless it is a special day. Feel free to post if you have more on this. Also, I hope this didn't "spoil" anything for anyone. ._.
EDIT: The Japanese server trend seems to be Half-Price-->More Instrument Items-->Half-Price-->More Food Items-->More Material Items-->restart cycle
And every day in between those days seems to be a day where there is no special. This seems as if it may hold true for us as well, since the last special I observed was the "More Instrument Items" and before that was Half-Price.
Market Calendar
09/29 - Half-Price Day
09/30 - No Special
10/01 - Food Day
10/02 - No Special
10/03 - Material Day
10/04 - No Special
10/05 - Half-Price Day
10/06 - No Special
10/07 - Instrument Day
10/08 - No Special
10/09 - Half-Price Day
10/10 - No Special
10/11 - Food Day
10/12 - No Special
10/13 - Material Day
10/14 - No Special
10/15 - Half-Price Day
10/16 - No Special
10/17 - Instrument Day
10/18 - No Special
10/19 - Half-Price Day
10/20 - No Special
10/21 - Food Day
10/22 - No Special
10/23 - Material Day
10/24 - No Special
10/25 - Half-Price Day
10/26 - No Special
10/27 - Instrument Day
10/28 - No Special
10/29 - Half-Price Day
--------------------------------
Instrument Day Items
Glowfly Fishing Bait (?)
Steel Pickaxe
Great Bugnet
Gunpowder
Food Day Items
Honey
Sleepyfish
Burst Arrowana
Scatterfish
Material Day Items
Ivy 75
Godbug 420
Nitroshroom 60
Toadstool 75
Blue Mushroom 24
Sm Bone Husk 30
Cactus Flower 100
Paintberry 60
Power Seed 280
Armor Seed 220
Sm Monster Bone 100
Wyvern Fang 60
Wyvern Claw 180
Rathian Scale 4900
Rathalos Scale 4900
Monster Fluid 5000
(?) Items are unconfirmed and some may only appear after a certain Hunter's Rank
*some of the items may not be listed with their proper names