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.
The Darkening of Namjan Forest (Quests of Doom 1 Volume 2)
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.
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 a buck! 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).
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).
Horror at Havels Cross
Similar in spirit to Lord of White Field, this level 2 adventure written for Basic D&D (5e) also has ghoulish entanglements.
The Wolves of Welton
A "basic" starter quest that may be more than meets the eye. PWYW for levels 2 to 3.
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.
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.
A unqiue take on a "haunting" with lots of role-playing potential. For levels 3 to 5.
A Haunting in Brenton
While I have not (yet) read through this adventure, it's a PWYW so you can try before you buy. For level 7 PCs.
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!)