DayOutput

days = {
  Monday: {},
  Tuesday: {},
  Wednesday: {},
  Friday: {},
  Sunday: {}
}

const WEEK = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']

const result = [[]];

let lastIndex

for (let day in days) {
  const currentIndex = WEEK.findIndex(d => d === day);
  if (!lastIndex || lastIndex + 1 === currentIndex) {
    result[result.length - 1].push(day)
  } else {
    result.push([day])
  };

  lastIndex = currentIndex;
}

console.log(
  result.map(group => {
    if (group.length > 1) {
      return [group[0], group.pop()].join('-')
    }

    return group
  }).join(' and ')
)