Well, you are in luck. There are several 1 to 2 session length adventures that can add a spooky side trek into your current campaign. Here are a few quickies that might work out. A couple of these might take a bit of translation for D&D 5th Edition, but the work will be well worth the fun.
This free adventure from Wizards of the Coast is the opening volley for the Curse of Strahd adventure path. However, even if you don't plan on running a Barovia game, you can modify some of the elements to be able to drop it into any campaign. As a one-shot, it works wonderfully since you can playing it like a DCC "funnel" adventure and kill your PCs with impunity.
Dead by Dawn - Dungeon Magazine, issue #176 (and 5e conversion)
Originally written for D&D 4th edition, this Dawn of the Dead hommage takes place in a ruined temple where the PCs must hold out until dawn while wave after wave of under attempt to break in and devours the poor souls. An 5th Edition adventure conversion guide (written by yours truly) costs less than 2 bucks! Conversion for level 3 PCs. This was also a big hit with the group, and I tied it into the Namjan Forest story line. The key is to keep the pressure on. It's a timer encounter, and each time the clock ticks, things just keep getting a bit worse.
Lord of the White Fields - Dungeon Magazine, issue #184
Also originally a 4th edition adventure, this side trek explore what happens when a sliver the Ghoul King's power leaks into a small village. This is another zombie... er... ghoul apocalypse style adventure that make the PCs solve the mystery before the realm is overrun by ghouls. This was also a lot of fun to run when my group played 4e. This has not been converted to 5e, but a quick conversion using ghouls right out of the Monster Manual would likely create an adventure for about levels 4 to 6 (give or take).
Horror at Havels Cross
Similar in spirit to Lord of White Field, this level 2 adventure written for 5th Edition Basic D&D, also has ghoulish entanglements.
The Chapel on the Cliffs
An army of skeletons drives out explorers of this ruined, cursed village. What secret are they guarding? Scaleable for PCs levels 2 through 6.
Frog God Game's excellent Quests of Doom series contains this excellent gem of an an adventure set in a forest overwhelmed by elemental shadow. The PCs must find the source of the infection from the Plane of Shadow. Watch gleefully when they crap their pants over the strength-draining shadow creatures. For PC levels 3 to 5 and 5th Edition compatible. I ran this for my group and it was a real hit. With a little more sketching out, this can turn into a mini-campaign arc.
The House of Poe
A creepy horror adventure for level 4 - 6 PCs where the adventurers explore a manor house at the edge of town. The adventure is filled with hidden references and homages to the works of Edgar Allan Poe making it a perfect Halloween adventure.
Death in the Cornfields
A level 4 vignette encounter that has surprising legs, despite its short length (and for only $1!). This is an excellent side trek that can be dropped into almost any session in the countryside.
The Witch of Underwillow
When wolves drag a child into the forest, there is only one option: find brave adventurers to follow the wolves and save the child! A short adventure for levels 1 to 3.
The Ghost and the Peddler
A unqiue take on a "haunting" with lots of role-playing potential. For levels 3 to 5.
The Wolves of Welton
A "basic" starter quest that may be more than meets the eye. PWYW for levels 2 to 3.
The House of the Midnight Violet
For levels 4 to 6, the PCs uncover clues to the lives of the former occupants as they explore this creepy mansion.
The Secret of Karnov Mansion
Dinner party during the full moon? What could possibly go wrong? For adventurer levels 4 to 6.
While I have not (yet) read through this adventure, it's a PWYW so you can try before you buy. For level 7 PCs.
Night of the Walking Dead
Noted in the comments by one of my readers, Night of the Walking Dead is a classic zombie horde adventure written originally for AD&D 2nd Edition. This adventure would be fairly easily translated to 5th Edition with a straight conversion using the D&D 5th Edition Monster Manual and still have the roughly the same encounter balance. For levels 1 to 3 (perhaps leaning toward the higher side of that range).
The Haunting at Velston
A free 2-encounter side trek from Kobold Press. Impactful and emotional.
Swamp Witch Monster Party
Not an adventure per se, but a pre-loaded encounter group for dropping into any campaign or overland travel. Encounter for level 3 PCs. PWYW.
Some ideas for incorporating spooky non-combat haunt effects into your game. PWYW.
There are a few others tagged with the "Halloween" keyword on DM's Guild. I expect several others will show up before the week's end.
Happy Haunting and please support the PWYW publishers!
(Oh, and please add your own suggestions in the comments!)