JSFiddle - React, Tailwind, and code Playground

JavaScript

let arrData = [
    {
      "dayOfTheWeek":"Monday",
      "isOpen":true,
      "hoursOfBusiness":[
         {
            "opensAt":"10:00:00",
            "closesAt":"15:00:00"
         }
      ]
    },
    {
      "dayOfTheWeek":"Tuesday",
      "isOpen":true,
      "hoursOfBusiness":[
         {
            "opensAt":"10:00:00",
            "closesAt":"15:00:00"
         }
      ]
    },
    {
      "dayOfTheWeek":"Wednesday",
      "isOpen":true,
      "hoursOfBusiness":[
         {
            "opensAt":"10:00:00",
            "closesAt":"15:00:00",
         }
      ]
    },
    {
      "dayOfTheWeek":"Thursday",
      "isOpen":true,
      "hoursOfBusiness":[
         {
            "opensAt":"09:00:00",
            "closesAt":"16:00:00",
         }
      ]
    },
    {
      "dayOfTheWeek":"Friday",
      "isOpen":true,
      "hoursOfBusiness":[
         {
            "opensAt":"10:00:00",
            "closesAt":"14:00:00",
         }
      ]
    },
    {
      "dayOfTheWeek":"Saturday",
      "isOpen":false,
      "hoursOfBusiness":[]
    },
    {
      "dayOfTheWeek":"Sunday",
      "isOpen":false,
      "hoursOfBusiness":[]
    }
]


function openingHours(data){
  let merged = [];
  let idx = -1;
  for (let i = 0; i < data.length; i++) {
    let day = data[i];
    if (
      idx == -1 ||
      merged[idx].hoursOfBusiness.opensAt != day.hoursOfBusiness.opensAt
    ) {
      merged.push({
        days: day.dayOfTheWeek,
        opensAt: day.hoursOfBusiness.opensAt,
        closesAt: day.hoursOfBusiness.closesAt,
      });
      idx++;
    } else {
      merged[idx].days.push(day.dayOfTheWeek);
    }
  }
  return merged;
}

console.log(openingHours(arrData))