The changing of the guards takes place on Friday noon only when QMII is in residence there. (Marselisborg too, I think.) And with a reduced band from the Music Corps of the Royal Lifeguard Regiment.
That makes it possible for these professional musicians to have their three weeks of summer holiday over the course of about eight weeks from late June to Early August.
That does not apply to fifers and drummers though, they are conscript signalmen. Not in the modern definition of course.
Back when the Royal Lifeguard Regiment was a regiment of the line, (*) grenadiers to be exact, drummers were placed next to the commanding officer of a unit, to drum the signals to the unit. With the fifer being a reserve. - Signalmen were then as now vital for commanding a unit in action.
The music corps doubled as stretcher bearers. Often wearing uniforms that were "reversed" (say red tunics instead of blue, with blue trousers instead of red) to their regiment, marking them as non-combatants. And as such that reversal of their uniform would often protect them against enemy fire.
(*) I.e. a regular combat regiment fighting in formation, typically a battle line for the infantry.
That in contrast to specialized jäger regiments or fusiliers who would more often fight in open formation as say skirmishers or providing flank security and the other countless jobs such regiments had.
Not to mention militia regiments, who were not professional, but called up when needed.