An interesting way to store 2 days of the week in a 64 bits.

dayOfWeek.js

// lookup <- one time cost
let dict = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']

// gives the two days based on the number
const getDay = (num) => ([(num % 7), (num / (7 - (num % 7)))]).map((val) => dict[val]);
// based on the days as mentioned above
const getNum = (arr) => [1]
  .concat(arr.map((val) => dict.lastIndexOf(val)))
  .reduce((acc, val) => (acc * val));