user.js

'use strict';
module.exports = (sequelize, DataTypes) => {
  const User = sequelize.define('User', {
    email: DataTypes.STRING,
    firstName: DataTypes.STRING,
    lastName: DataTypes.STRING,
    companyId: DataTypes.INTEGER
  }, {});
  User.associate = function(models) {
    User.belongsTo(models.Company, {foreignKey: 'companyId', as: 'company'})
    User.belongsToMany(models.WorkingDay, {through: 'UsersWorkingDays', foreignKey: 'userId', as: 'days'})
  };
  return User;
};

usersworkingday.js

'use strict';
module.exports = (sequelize, DataTypes) => {
  const UsersWorkingDay = sequelize.define('UsersWorkingDay', {
    userId: DataTypes.INTEGER,
    workingDayId: DataTypes.INTEGER
  }, {});
  UsersWorkingDay.associate = function(models) {
    UsersWorkingDay.belongsTo(models.User, {foreignKey: 'userId'})
    UsersWorkingDay.belongsTo(models.WorkingDay, {foreignKey: 'workingDayId'})
  };
  return UsersWorkingDay;
};

workingday.js

'use strict';
module.exports = (sequelize, DataTypes) => {
  const WorkingDay = sequelize.define('WorkingDay', {
    weekDay: DataTypes.STRING,
    workingDate: DataTypes.DATE,
    isWorking: DataTypes.BOOLEAN
  }, {});
  WorkingDay.associate = function(models) {
    WorkingDay.belongsToMany(models.User, {through: 'UsersWorkingDays', foreignKey: 'workingDayId', as: 'employes'})
  };
  return WorkingDay;
};