State the transaction date whereby DigixDAO was transacted at the hightest price.
the highest price refers to max(price); DigixDAO refers to name = 'DigixDAO'
SELECT T2.date FROM coins AS T1 INNER JOIN historical AS T2 ON T1.id = T2.coin_id WHERE T1.name = 'DigixDAO' ORDER BY T2.price DESC LIMIT 1