How to JOIN by date columns?
If you aren't sure that all of your dates in the second table end are on the first day, you can be sure the comparison on year/month with EXTRACT( YEAR_MONTH FROM datecol) like this,

SELECT
  t2.Day,
  t1.Month,
  t1.Value1,
  t2.Value2
FROM Table2 t2
LEFT JOIN Table1 t1
  ON EXTRACT(YEAR_MONTH FROM t2.Day) = EXTRACT(YEAR_MONTH FROM t1.Month)


It is likely faster regardless, unless you have an index on t1.month