Assume that you have 31 sheets, named 1 to 31 respectively.
This script works when the spreadsheet is opened by using a simple onOpen trigger.
new Date(): gets today's date
now.getDate(); is a shorthand way of getting the day of the month. This is a number
ss.getSheetByName(sheetname);: assigns the day number to the sheet name
For some reason, Tanaike's answer keeps giving me an error when running the line when it gets the blob using DriveApp. It only worked after I changed that line to get the file using
var img = UrlFetchApp.fetch(fileUrl).getBlob();
Just in case it helps someone from google like me.