[Xrystal]

Okay, found a function in the wow files that I didn't see in wowprogramming and must have missed from wowpedia, and it all seems to work apart from your main requirement ... updating the MotD text automatically. Here's the code anyway. Hopefully Phanx can see what is wrong.

-- Get the localised addon information
local addonName,addonData = ...
addonData.MotD = ""
 
addonData.CheckGuildEvents = function()
    addonData.NumGuildEvents = CalendarGetNumGuildEvents()
    if ( addonData.NumGuildEvents == 0 ) then return end
    local month, day, weekday, hour, minute, eventType, title, calendarType, textureName
    for i = 1,addonData.NumGuildEvents do
        month, day, weekday, hour, minute, eventType, title, calendarType, textureName = CalendarGetGuildEventInfo(i);
        thisInfo = tostring(i) .. ": " .. title .. " at " .. GameTime_GetFormattedTime(hour, minute, true) .. " on " .. string.format(GUILD_NEWS_DATE, CALENDAR_WEEKDAY_NAMES[weekday], day, month)
        -- GuildSetMOTD(thisInfo)               -- This works
        if ( strlen(addonData.MotD) + strlen(thisInfo) > 128 ) then 
            -- GuildSetMOTD(addonData.MotD)     -- This doesn't work, wipes out old MotD but this doesn't appear in its place
            break
        else
            addonData.MotD = string.format("%s\n%s",addonData.MotD, thisInfo)
            -- GuildSetMOTD(addonData.MotD)     -- This doesn't work, wipes out old MotD but this doesn't appear in its place
        end
    end
    
    print(addonData.MotD, "(", strlen(addonData.MotD), ")")
    GuildSetMOTD(addonData.MotD)                -- This doesn't work, wipes out old MotD but this doesn't appear in its place
    --GuildSetMOTD("Testing testing 1 2 3")     -- This works
end
 
local function OnEvent(self,event,arg1,arg2,arg3,arg4,arg5)
    if ( event == "ADDON_LOADED" and arg1 == addonName ) then
        if not IsAddOnLoaded("Blizzard_Calendar") then
            LoadAddOn("Blizzard_Calendar")
        end
    elseif ( event == "PLAYER_ENTERING_WORLD" ) then
        local weekday,month,day,year = CalendarGetDate()
        local month, year, numDays, firstWeekday = CalendarGetMonth(0)
        CalendarSetAbsMonth(month,year)     
        OpenCalendar()
    elseif ( event == "CALENDAR_UPDATE_GUILD_EVENTS" ) then
        addonData.CheckGuildEvents()
    elseif ( event == "CALENDAR_UPDATE_EVENT_LIST" ) then
        addonData.CheckGuildEvents()
    end
end
 
local XGMotDFrame = CreateFrame("Frame","XGMotDFrame",UIParent)
XGMotDFrame:SetScript("OnEvent", OnEvent)
XGMotDFrame:RegisterEvent("ADDON_LOADED")
XGMotDFrame:RegisterEvent("CALENDAR_UPDATE_GUILD_EVENTS")
XGMotDFrame:RegisterEvent("PLAYER_ENTERING_WORLD")
XGMotDFrame:RegisterEvent("PLAYER_GUILD_UPDATE")
XGMotDFrame:RegisterEvent("CALENDAR_UPDATE_EVENT_LIST")

CALENDAR_UPDATE_EVENT_LIST is triggered everytime you reload the UI and CALENDAR_UPDATE_GUILD_EVENTS is triggered when you first log in and when new guild events are added.