Automating Google Spreadsheets – Email Reminders - google-apps-script

Question

I have a Google Spreadsheet and have modified the code below as indicated to send an email alert 7 days before the date condition is due. This works perfectly, however it only looks at the first sheet and the spreadsheet contains 23 sheets in total. I think I need to include an array and loop the code but cannot work this out so would appreciate some help! Thanks in advance.
function checkReminder() {
var spreadsheet = SpreadsheetApp.getActiveSpreadsheet();
// set the first sheet as active
SpreadsheetApp.setActiveSheet(spreadsheet.getSheets()[0]);
// fetch this sheet
var sheet = spreadsheet.getActiveSheet();
// figure out what the last row is
var lastRow = sheet.getLastRow();
// the rows are indexed starting at 1, and the first row
// is the headers, so start with row 2
var startRow = 7;
// grab column 14 (the 'days left' column)
var range = sheet.getRange(7, 14, lastRow - startRow + 1, 1);
var numRows = range.getNumRows();
var days_left_values = range.getValues();
// Now, grab the reminder name column
range = sheet.getRange(7, 15, lastRow - startRow + 1, 1);
var reminder_info_values = range.getValues();
// Now, grab the deceased name column
range = sheet.getRange(3, 1);
var name_of_sheet = range.getValue();
var warning_count = 0;
var msg = "";
// Loop over the days left values
for (var i = 0; i <= numRows - 1; i++) {
var days_left = days_left_values[i][0];
if (days_left == 7) {
// if it's exactly 7, do something with the data.
var reminder_name = reminder_info_values[i][0];
msg = msg + "Reminder: " + name_of_sheet + " inscription work is due in " + days_left + " days.\n";
warning_count++;
}
}
if(warning_count) {
MailApp.sendEmail("my#email.com",
"Reminder Inscription Schedule Message", msg);
}
};