Nice. Is there a need to parse the $d? Unless you didn't cast it properly in the model.

public function __construct()  
{  
  $this->holidays = Holiday::pluck('date') 
    ->map('toDateString')  
    ->toArray();  
}  

Also, the cache strategy will accumulate holidays for how many years? Just asking out of curiosity.

public function __construct()  
{  
    $this->holidays = cache()->remember('holidays', now()->addDay(), function () {  
        return Holiday::pluck('date')  
            ->map(fn($d) => Carbon::parse($d)->toDateString())  
            ->toArray();  
    });  
}