//Рабочий код
std::vector<Month*> date_arr;
Month* arr_ptr = new Month[12];
for (int j = 0; j < 12; j++) {
    arr_ptr[j] = new Month(constants::months[j], is_leap_year);
}
date_arr.push_back(arr_ptr);

//Вызов этого вектора
auto tmp = date_arr[0][0].days_vec[0];

//Month, если нужно
struct Month
{
    Month(){}

    Month(QString month, bool is_leap_year)
    {
        if (is_leap_year) {
            days_count = constants::months_days_map_ly.at(month).second;
        } else {
            days_count = constants::months_days_map.at(month).second;
        }

        for (int i = 0; i < days_count; i++) {
            days_vec.push_back(new Day());
        }
        day_txt = QString::number(days_count);
    }

    Month* operator =(Month* other)
    {
        days_count = other->days_count;
        days_vec.swap(other->days_vec);
        return this;
    }

    ~Month()
    {
        for (size_t i = 0; i < days_vec.size(); i++) {
            delete days_vec[i];
        }
    }

    //Variables
    std::vector<Day*> days_vec;
    int days_count;
    QString day_txt;
};