Tony Vargas
Legend
A lone assassin? OK. A very sensible city encounter. The party has made an enemy, the enemy send someone to deal with them...A lone assassin in a town can be a deadly encounter and yet it has little to no implications (beyond the basic ones of a fantasy world) in your worldbuilding so use that instead of the Dire Wolves.
...Then do it at least twice more before the party can sleep that night to provide a suitable challenge?
'Must have 6-8 encounters every single calendar day' is nonsense, and would lead to parties going from 1st-20th in about a calendar month.I've asked the same question. The only possible answer I've been able to glean is that some people feel that in every day they are in any way active... PC's must have an adventuring days worth of encounters...
I think the bar is simply any given day that they do have one encounter, you need to provide some more to fill it out, or it's a "5MWD." Such days would presumably be sprinkled among down-time and un-eventful days. It's a constraint on encounter/adventure/campaign design, unless you change of finesse the rules to fit the guideline to campaign, instead of change/finesse the campaign to fit the guideline.
You could quite plausibly have one encounter with a lone assassin sent to kill you, no matter how safe the area. That encounter would break the guidelines, providing a 5MWD. It wouldn't exactly make a lot of sense for the enemy with resources to send many assassins against you to spread them out to provide a challenge, rather than overwhelm and destroy the party all at once.But you wouldn't have ANY encounters in a safe area... that's the thing. It's a safe area.... easy, hard, moderate none would be there in a safe area.

(But, /anything/ can be rationalized: for instance, the enemy in question puts out a 'hit' on the party, a bounty on their heads if killed within 24 hrs, say - and various dis-organized would-be assassins take stabs(npi) at collecting said bounty, some individually, then seeing that failure, the remainder, perhaps, as a hastily-assembled team...
...hey, look, it's time pressure again, this time on the other side!)

Why would it matter? Clearly, if you're willing to subordinate your world to the needs of the guidelines, by always having 6-8 (or 3-18, though the extremes will have lesser issues of their own) encounters on days that you have any encounters at all, you can do so.Can you provide examples from the game you're currently running?
It is odd if you stop and think about it, but hardly un-heard-of. It creates a sense of consistency, if only for the DM. You can re-enforce to yourself that you're being 'impartial' and running a 'real world,' by letting the dice decide things, even things that don't 'directly' matter to the PCs, but may indirectly matter to them via the shaping of the campaign world, I suppose.Just seems to me like something very odd to do....make rolls to determine something where no one will be impacted directly by the randomness of the roll.
On the WotC boards (and I think here), in the brief 4e era, the practice of having monsters or NPCs roll off-screen or when fighting eachother was dubbed 'monsterbating.' The pointlessness of the activity becomes clear when the system uses different stats and assumptions for monsters/NPCs than for PCs - it reminds you that the game is about your players, not your world. (Of course, an Empowered DM can make the game about the campaign, not the players - sometimes called 'setting tourism').

In theory. In practice they'll have a different impact on the party (class balance & encounter difficulty) and paint a different picture of the campaign.So, aren't 3 deadly encounters the equivalent of 18 easy encounters?
In 3 deadly encounters, for instance, a barbarian with 3/rages per day is going to be a star performer, being at peak performance in all three, and with peak performance being desperately needed from everyone because the encounters are individually quite dangerous. In the 18-easy-encounter day, it'll hardly seem worth it to rage in any of those encounters, and the barbarian's performance won't stand out. Assuming the slow attrition of those 18 encounters adds up, he might rage the last three, say.