Here's the most recent information being passed around BG. Seems to have a good amount of evidence supporting it.

For yellow and blue, the time and day the mob is popped or claimed apparently determines its weakness.

Attempting to proc a !! while stunned from a Red !! will not work
Attempting to proc !! while the monster is stunned using a TP move/Casting will not work
Attempting to proc !! on a stunned monster that was only auto attacking (sans iron giant attacks) WILL work


YELLOW (DAY BEFORE + CURRENT DAY + DAY AFTER):
(Magian Item/Seals/+2 items)

Lightsday: Holy, Banish 2, Banish 3, Banishga 2, Flash, Dark Threnody, Radiant Breath
Darksday: Bio 2, Aspir, Dispel , Drain, Kurayami:Ni, Light Threnody, Eyes On Me
Firesday: Fire 3, Fire 4, Firaga 3, Flare, Katon:Ni, Ice Threnody, Heat Breath
Earthsday: Stone 3, Stone 4, Stonega 3, Quake, Doton:Ni, Lightning Threnody, Magnetite Cloud
Watersday: Water 3, Water 4, Waterga 3, Flood, Suiton:Ni, Fire Threnody, Maelstrom
Windsday: Aero 3, Aero 4, Aeroga 3, Tornado, Huton:Ni, Earth Threnody, Mysterious Light
Icesday: Blizzard 3, Blizzard 4, Blizzaga 3, Freeze, Hyoton:Ni, Wind Threnody, Ice Break
Lightningsday: Thunder 3, Thunder 4, Thundaga 3, Burst, Raiton:Ni, Water Threnody, Mind Blast

BLUE (INCREASED RARE ITEM DROP - Does NOT make it 100%):

6:00-14:00 - Piercing
Dagger: Shadowstitch(70), Dancing Edge(200), Shark Bite(225), Evisceration(230)
Polearm: Skewer(200), Wheeling Thrust(225), Impulse Drive(240)
Archery: Sidewinder(175), Blast Arrow(200), Arching Arrow(225), Empyreal Arrow(250)
Crossbow: Slug Shot(175), Blast Shot(200), Heavy Shot(225), Detonator(250)

14:00-22:00 - Slashing
Sword: Vorpal Blade(200), Swift Blade(225), Savage Blade(240)
G.sword: Spinning Slash(225), Ground Strike(250)
Axe: Mistral Axe(225), Decimation(240)
G.axe: Full Break(225), Steel Cyclone(240)
Scythe: Cross Reaper (225), Spiral Hell(240)
Katana: Blade: Ten(225), Blade: Ku(250)
G.katana: Tachi: Gekko(225), Tachi: Kasha(250)

22:00-6:00 - Blunt
H2H: Raging Fists(125), Spinning Attack(150), Howling Fist(200), Dragon Kick(225), Asuran Fists(250)
Club: Skullbreaker(150), True Strike(175), Judgement(200), Hexa Strike(220), Black Halo(230)
Staff: Heavy Swing(5), Shell Crusher(175), Full Swing(200), Spirit Taker(215), Retribution(230)

RED (ATMA / KEY ITEM DROP):

Dagger : Cyclone / Energy Drain
Sword : Red Lotus Blade / Seraph Blade
Great Sword : Freezebite
Scythe : Shadow of Death
Polearm : Raiden Thrust
Katana : Blade: Ei
Great Katana : Tachi: Jinpu / Tachi: Koki
Club : Seraph Strike
Staff : Earth Crusher / Sunburst