SELECT "Date(s) conducted" FROM table_52880 WHERE "Lead" = '8.6%'
On what date was the lead 8.6%?
CREATE TABLE table_52880 (
    "Date(s) conducted" text,
    "Cons" text,
    "Plaid Cymru" text,
    "Lib Dem" text,
    "UKIP" text,
    "Green" text,
    "Others" text,
    "Lead" text
)