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find directory -type f -name "*.html" -exe sed -i.old 's|10/31|11/30|g' {} \;

If your sed don't support -i option, replace by perl

find directory -type f -name "*.html" -exec perl -p -i.old -e...

Try:
for f in *; do
cp -p "$f" "$f.old" && sed 's|10/31|11/30|g' "$f.old" > "$f"
doneor GNU:
sed -i.old 's|10/31|11/30|g' *