My son went through a similar and very difficult period at daycare around the same age. Anecdotally, it seemed to be related to three factors:
When he changed classes, he went from a very strong attachment with
his teacher, to a teacher he didn't know and didn't 'click' with. (She was a perfectly
fine teacher, they just didn't have a strong rapport, he missed the
teacher that he was very close with in the previous class.)
He had some time off school for an extended vacation and over the
holidays, and preferred to remain on vacation rather than go back to
daycare, didn't understand why we had to stop doing that nice stuff and go back to school
He was going through a developmental stage that involved more
boundary-pushing, frustration, defiance and aggression naturally
In your case I would venture that it's quite a bit related to the second part-- the 4 weeks of being near you all day were so much more desirable than being separated from you at daycare, that your little one is strongly protesting the change. A month is such a long time in the life of a toddler, he was probably sure that holiday was the new normal, and understandably unhappy to find out that was not the case!
This will probably improve with time and getting back into the daycare routine, but in my son's case he didn't fully return to his happy self until he moved to the next class, and away from the teacher he didn't connect with, and associated with his distress.
While he had to be in the class where he wasn't happy, I did take some steps to make the most of the situation. I had a conference with the head of the daycare, and his current and former teachers. They were able to make some personalized recommendations to help him (for example, I dropped him off in the office and the owner of the daycare walked him to class for a few days, to break the anxiety/habit of melting down when I left him in the new classroom.) I also did my best to maximize connection and minimize demands on him at home. A defiant, aggressive toddler can be boundary testing, but they can also just be filled with bad feelings they don't know how else to express. I tried to help him decompress with sensory playtime and cuddles as soon as we got back from daycare. We started using some little rituals for parting and leaving, like looking for ducks on the pond we pass on the way in to daycare, and singing a song on the way home, to help him feel loved, add consistency to his day, and replace the unhappy habit of crying on the way to school with a routine he looked forward to.
I think the best thing you can do is talk to his teachers about what can be done to support him in this tough transition. Don't hesitate to ask for accommodations if you can pinpoint something specific about school that is causing him stress or if you know of something simple that could help (like dropping off in his old classroom, listening to music, permission to have a comfort toy from home, etc).