It's every day when the classes are balanced.
Classes dont have to be balanced
every day. No-one is saying they do, and no-one is saying 'thou must enforce 6-8 encounters and 2-3 short rests on your party via doom clocks every adventuring day.
Stop. Read.
Let me show you an example of some adventures taking a group of PCs going from 5th to 6th level. A total of 4 adventuring days:
On returning from their last quest, where they advanced to 5th level (and resting up and sorting out admin) they're approached by a High priest they've previously had interactions with and are friendly too. His divinations have revealed that an enemy they thought was vanquished several levels ago, has come back from the dead as a foul undead monster, and seeks to finish the evil he started in life! The PCs track him down to a dungeon (taking a few days, and with one of those days featuring a Deadly random encounter (
AD 1)), the entrance of which is a mysterious portal. Entering, they find it's a one way magical gate, trapping them within!
Unbeknown to the PCs the entire dungeon is a trap, designed by their old foe to suck the life force out of them and kill them, sucking their energy and syphoning it to himself. Every hour that passes, the PCs must make a DC 10 Con save, or gain 1 level of exhaustion, and lose 1 spell slot of the highest level they know as the eldritch energies drain magical and life force from them. For every 5 failed saves, the BBEG gains 1 legendary action and 1 more legendary save.
[the dungeon has 6-8 encounters in it, with a final showdown with the BBEG at the end] (
AD 2)
Returning from the dungeon, the PCs find that the High Priest has mysteriously disappeared. Later that evening, one of the PCs receives a 25 word message from the Priest, cast via a
Sending spell (he found a bit of copper wire in his cell, before being knocked unconscious by the guards and bound). He relays that he has been captured by an evil cult, who plan to sacrifice him in 3 days time at the Midwinter solstace, and gives the PCs a clue about where to find him.
On their way to the cult lair, they get a Deadly random encounter on one of the days (
AD 3) before reaching the cult lair just in time to raid it and save him before the ritual is complete (9 encounters, time for 3 short rests) (
AD 4).
That's 4 adventuring days, with the Caster likely shining on days 1 and 3 and the Martials likely shining on days 2 and 4.
Overall balance is maintained, and the spotlight moves from PC to PC. Everyone has a chance to shine, the stakes are high, nothing is contrived, and the adventure is propelled forward with impetus and win/lose effects in order.