The worst day 1917 came on 3rd May 1917 when 7,371 died, of whom 4,968 are listed on the Arras memorial. The battle of Arras lasted from the 9th April to the 16th May (see more here: Battle of Arras Wikipedia).
This graph shows the highs and lows of the year:
The worst fighting that year was focused on Ypres with Tyne Cot and the Menin Gate berain 50,526 names for 1917. Not far behind were Arras (23,341) in the Pas De Calais. Other cemetery numbers were then about 5,000 or lower. So you can see where the main fighting grounds were.