CREATE TABLE table_78209 ("Date ( s )  conducted" TEXT, "Polling organisation/client" TEXT, "Cons" TEXT, "Plaid Cymru" TEXT, "Lib Dem" TEXT, "Others" TEXT, "Lead" TEXT)

Tell me the dates conducted for plaid cymru of 19%
SELECT "Date(s) conducted" FROM table_78209 WHERE "Plaid Cymru" = '19%'
SELECT "Date(s) conducted" FROM "table_78209" WHERE "Plaid Cymru" = '19%'