Vitamins are a very tricky thing and there’s a lack of good quality research on them (and they aren’t regulated in the same way that pharmaceuticals are so this complicates the matter.)

Vitamin D is important for brain function and being deficient in it could well have impacted your mood but the same can be said for B vitamins. There’s less research on magnesium and zinc but it’s possible (I’d say probable) that deficiency in these could impact your mood. The other thing that I’d be considering is iron deficiency because it’s really pernicious and while it may or may not have a direct impact on your mood the symptoms of iron deficiency can absolutely mirror the symptoms of depression.

Supplementing iron in the case of deficiency usually takes a couple of weeks for its effects to be noticed so if it was only a matter of days and you found a significant improvement in your mood then I’d be inclined to rule out anaemia unless there are other symptoms which indicate it.

As per this study (Novel Targets for Fast Antidepressant Responses: Possible Role of Endogenous Neuromodulators), if your multivitamin contains vitamin C then this could be having a positive impact on your mood as Vitamin C appears that it may be a fast-acting agent that works similar to antidepressants (speaking in broad terms here). (Also note that magnesium is an NMDA antagonist as well so that could be causing the same effect.)

What’d be really difficult to determine is whether it’s just one vitamin/mineral that’s having this effect or whether it’s a combination.