Label all gmail conversations you were having on this day in the past.

onthisday.gs

// This function finds all the conversations you were having on this day in the past years (since Gmails release in 2005),
// and adds the "On This Day" label to them. Put it in a new apps script project and set it to run at 3am every day.
// It'll first clear out the label and then add all relevant messages.
function onThisDay(){
  var onThisDayLabel = GmailApp.getUserLabelByName("On This Day")
  var date = new Date()
  var yesterday = new Date();
  yesterday.setDate(yesterday.getDate() - 1);
  var tomorrow = new Date();
  tomorrow.setDate(tomorrow.getDate() + 1);
  if (onThisDayLabel) {
    Logger.log('label found')
    // remove all the current messages on the label
    Logger.log('removing all current messages on the label')

    var threadsOnLabel = onThisDayLabel.getThreads()
    for (yy = 0; yy < threadsOnLabel.length; yy++) {
      onThisDayLabel.removeFromThread(threadsOnLabel[yy])
    }

    Logger.log('finding messages for each year')
    for (var xx = 2005; xx < date.getYear()-1; xx++) {
    Logger.log('looking at year ' + xx)
    tomorrow.setFullYear(xx)
    yesterday.setFullYear(xx)
    var searchQuery = "to:me  -category:updates -category:promotions -category:social after:" + Utilities.formatDate(yesterday, "GMT", "YYYY/MM/dd") + " before:" + Utilities.formatDate(tomorrow, "GMT", "YYYY/MM/dd")
    Logger.log(searchQuery)
    var thisYearsThreads = GmailApp.search(searchQuery)
    for (var j = 0; j < thisYearsThreads.length; j++){
      thisYearsThreads[j].addLabel(onThisDayLabel)
    }
    }

   
  } else {
    Logger.log('label not found')
    
  }
}