According to the Wikipedia entry, they celebrate and perform rituals in the following manner:
Hextorian services feature chanting, wind instruments sounding
discordant notes, shouts and screams, and iron weapons
striking against solid objects.
Hextorians swear mighty oaths in battle, such as "Strength in
victory!" "Mercy is for the weak!" or "Death to the unworthy!" a
not-so-veiled reference to Heironeous and his worshippers.
(Their rituals might sound something like the first 30 seconds of this)
It also mentions a possible way to gain favor by performing certain tasks:
The Feats of Strength
Before a Hextorian priest can advance in rank, they must demonstrate their martial prowess > and fitness to rule.
The nature of these feats varies, but they are all revealed through
prayer and divine revelation. They can vary from tests of endurance
that can be performed within the temple walls to unholy crusades
against the forces of Good.
As for the holidays, I was able to find one that you didn't mention, as well as some historical information on the ones you did:
Hextor’s faithful consider the Day of the Tyrant which falls on
Charder 24 as his primary holy day. The further subjugation of
oppressed people is a common game on this day, though the more
ambitious followers of Hextor seek to confront the unconquered
governments and further weaken their regimes, aiming towards a goal of
inflicting a Hextorian state in its place. [source]
Blooding. During the week of Growfest, Hextorians exhibit their strength, fitness, and martial prowess, sacrificing the lives of war prisoners to feed their god. Others cut themselves, offering their own blood to Hextor.
The second of the four festival weeks in the Greyhawk Calendar,
Growfest represents the beginning of Low Summer. The Spring Equinox is
the 4th of Growfest. The week is particularly sacred to Atroa, as well
as the Old Faith.
The elves call Growfest Springrite, and spend the week around the
equinox dancing and singing, involving themselves in nothing but
merriment. All important decisions and actions are postponed by the
elves until the week is over, with the significant exception that this
is when most elven couples marry or announce their betrothals.
The faithful of Hextor call this time the Blooding; they observe this
week with contents of strength, fitness, and skill, offering blood to
propitiate their god.
Each festival week is marked by the aquamarine moon Celene shining
full in the night sky.
Discordsmight. Celebrated on the 25th of Ready'reat, commemorates the struggle between the Medegian Bladelands and the followers of Heironeous. The church of Hextor teaches that this was a glorious victory in which the faithful pushed their boundaries all the way to the Flanmi River. Worshipers of Heironeous, however, celebrate a parallel holiday of Valormight on the same day, where they celebrate the fact that the Hextorians were unable to push past that river as a great victory for the forces of Good.
Medegia was originally settled during the Great Migrations period by
the Medegi sub-tribe of the Aerdi people. The Medegi worshiped the
entire Oeridian pantheon, but worshiped Hextor above all others. They
named the region the Medegian Bladelands, and, led by their war-leader
Kahabros, conquered to the banks of the Flanmi River.
Appalled by their success, followers of Heironeous fought a ten-month
war with the armies of the Bladelands, commemorated today by the
Heironeans in their holiday of Valormight, and by the Hextorians in
their holiday of Discordmight. The lengthy conflict weakened both
nations, allowing them to be conquered by the Kingdom of Aerdy by the
time of Grand Prince Mikar of Garasteth.
After 1 CY, Overking Nasran of Cranden appointed a cleric of Pholtus
to rule the region as Holy Censor, renaming it the See of Medegia. The
Holy Censor was considered the chief cleric of the Great Kingdom, a
powerful position indeed.
In 252 CY, Overking Toran II of Rax stripped the See of Medegia and
the office of Holy Censor from the Church of Pholtus and granted it to
the Church of Zilchus, allies of his royal house.
In 450 CY, after the Turmoil Between Crowns, Overking Ivid I passed
the office of Holy Censor and the rulership of Medegia to the church
of Hextor. There it remained for the duration of Medegia's existence
as a Holy See.
As the strength of the Overking on his Malachite Throne declined, the
power of the Holy Censor proportionately grew, and the See of Medegia
became virtually independent until 586 CY, when the Great Kingdom's
capital of Rauxes became engulfed in magical catastrophe and that
nation crumbled. In 587, much of the nation re-formed under the
leadership of Xavener of House Darmen, who eliminated the rival power
of the Holy Censor by recreating Medegia as a Marchland rather than as
a Holy See. This brings us to the present.
The Fist of Eternal Malachite. Celebrated on Midwinter's Day, this holiday commemorates the battles that helped Hextorians gain ascendancy during the Turmoil Between Crowns. In lands other than the former Great Kingdom, this day is a remembrance of battles fought in the name of the local ruler.
The Turmoil Between Crowns includes the decade of internal schisms in
the Great Kingdom during the reign of Overking Nalif, as well as the
civil war that followed the ascension of Ivid I.
After a century of misrule by the increasingly decadent Celestial
House of Rax, the other nobles of the Great Kingdom agreed that
another Rax ruler was unacceptable. The last royal heir of the house
was assassinated in 446 CY and Ivid I of the House of Naelax claimed
the throne, thus plunging the nation into civil war.
Most of the House of Naelax sided with Ivid, though a few of his
cousins did not. The church of Hextor enthusiastically supported
Ivid's cause, lending their church armies to the effort. House Cranden
opposed the Naelax prince, as did many in the house of Garasteth and,
of course, the remnants of House Rax. Nobles of all the Celestial
Houses used the war as cover to settle unrelated old scores with one
another, thereby increasing the discord of the era. Alliances shifted
until 449 CY, when House Darmen elected to back the Naelax under the
premise that it's always wise to side with the likely winner. The
Darmen troops sacked Rel Deven in Harvester of that year and helped to
secure the kingdom's central lands. In Planting of 450, the other
houses finally accepted Ivid as Overking, their leading princes paying
homage to their new ruler along the Great Way in the Parade of Crowns.
Much knowledge was lost during the Turmoil Between Crowns, including
the charts and records of the expeditions of House Atirr. As a
condition for accepting his rulership, the rulers of North Province,
South Province, and Medegia required Ivid to give them virtual
autonomy over their lands. House Cranden attempted to ally South
Province with the Iron League, but this ultimately failed. Most
priests of Heironeous left the kingdom or were marginalized in
isolated areas, speeding the Great Kingdom's decline into evil. The
fiend-summoning of House Naelax, unusual at the time of the Turmoil,
eventually became commonplace among the kingdom's nobles.