Help with If Statement with 'or'

Hi all again. I need to run this script only twice a month. I'm getting an error which I'm sure I have the wrong syntax. Can anyone help me?

WDAY=`date '+%d'`
if [[ $WDAY = "10" ]] -o [[ $WDAY = "25" ]]; then
  echo "Script Running day $WDAY, please wait..."
  date
else
  echo "Script Today  $WDAY - will not run"
  return
fi

Script2run.sh

You could try:

if [[ $WDAY = "10" ]] || [[ $WDAY = "25" ]]; then

or

if [[ $WDAY = "10" || $WDAY = "25" ]]; then

Thank you!