[Vrul]

You are referencing arg1 when it should be spellID and you are checking the wrong events. Give this a shot:

local SPELL_ID = 24275  -- Hammer of Wrath
 
local frame = CreateFrame("Frame")
frame:SetScript("OnEvent", function(self, event, spellID)
    if event == "SPELL_ACTIVATION_OVERLAY_GLOW_SHOW" then
        if spellID == SPELL_ID then
            print("Hello, WoW!")
        end
    elseif event == "SPELL_ACTIVATION_OVERLAY_GLOW_HIDE" then
        if spellID == SPELL_ID then
            print("Bye")
        end
    end
end)
frame:RegisterEvent("SPELL_ACTIVATION_OVERLAY_GLOW_SHOW")
frame:RegisterEvent("SPELL_ACTIVATION_OVERLAY_GLOW_HIDE")