const index = Math.floor(Date.now() / (3 * 864e5)) % 3; // 0, 1 или 2
console.log( ['X', 'Y', 'Z'][index] );3 * 60 * 60 * 24 * 1000 = 3 * 86400000 = 3 * 864e5for (let i = 0; i < 6; i++) {
console.log( i % 3 );
}
// 0 1 2 0 1 2