五月病って英語でなんて言うの？

Lassitude: a state of physical or mental weariness; lack of energy.
"She was overcome by lassitude and retired to bed."
If you are lacking energy and simply want to have a 'duvet day' then your symptom is lethargy where you have little energy. If you are not enthusiasticv about anything, then you may be feeling generally:"demotivated"

A lack of energy or motivation...ongoing inertia, inactivity, slowness, torpidity.
We can all get so used to doing nothing much that we become lethargic over time.
"Demotivated to the max" Lethargy :-))

1. Feeling lethargic after a vacation.
   Lethargic means sluggish, inactive, slow.

2. Lacking energy after my vacation.
   If you are lacking energy after a vacation it means that you are lazy and not motivated to get involved in new activities.

3. Post-vacation blues.
   The expression "post-vacation" means after a vacation.
   Blues means a feeling of sad and low spirits.

When returning from home after a long vacation from school or work, most people feel unmotivated to work or focus on school. Sometimes in the U.S. we call it the post-vacation blues. The term "blues" describes a sad or depressed mood.

I'm feeling so sluggish right now!

**Sluggish- is a term which means someone is very slow or very tired. Much like the animal as a slug moves very slowly.

I can't be bothered with anything today.

**Can't be bothered- when you don't want to deal with anyone or anything. A general lack of energy!

I'm so down after my holiday!

**'down' meaning I'm feeling down/sad/ unhappy after my holiday.

Hope this helps!! ^^