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Konsi's taking charge of a situation? This is a big move for her.
Slots work like MP; you don't equip the spells in them (with a high Wis score you can prepare more spells than you can cast, anyway) but use them to cast previously prepared spells. I like to visualise them as Spell Gates, through which you cast the spells.
So anyway it's very much possible to keep a slot or two unused!
Under the base rules of 5e, for clerics, you can only cast spells that you have prepared. (This includes, for example, ritual casting - you need to have the spell prepared to ritual cast it.)
The last paragraph provides the rule for when you're allowed to prepare spells - and that's "when you finish a long rest" - so under RAW you can leave a slot empty for the day, but you have no way to fill that slow until you finish a long rest.
Since you can't fill slots during the day, there's no real reason to hold an empty slot, outside of incredibly niche or weird situations like "I expect to be possessed later today, so I'm going to deliberately hobble my abilities in advance."
Of course, DMs may run house rules that change this - it'd be an interesting house rule to allow casters to leave open slots then prepare them during the day, but it's not permitted in RAW, I'm afraid.
Yeah, you can only prepare them when finishing your long rest.
But they don't get equipped into a slotv though...
Slots are essentially MP with levels.