//starting date
let dateStartTemp = sheet.getCell(1,0).getValue();
//how many days after Monday?  
let dateCalculator = (dateStartTemp - 2)%7;
//Date of the Monday of that week
let dateStart = dateStartTemp - dateCalculator;
//The row I want to search  
let findRow = rangeSummary.getRow(0);
  let findRowVal = findRow.getValues();
  let startCell = -1;
  for(let i=1; i<colCount; i++){
    if(findRowVal[0][i]===dateStart){
      startCell = i;
    }
  }
if(startCell === -1){
  console.log("could not find date!")
}