For this type of question it is a great idea to post CREATE TABLE statements for your tables and INSERT statements for your test data. Finally it helps to have the expected result given the test data. This makes it easy to copy and paste into a query window to develop a tested solution.
Since you did not do this, you will get an untested solution which is based on a guess of what you want.
SELECT t1.OrderNumber, t1.item, coalesce(t1.Date, t2.Date)
FROM table1 t1
LEFT JOIN table t2 ON t1.OrderNumber = t2.OrderNumber