It was taken on July of 1915, Nicholas' diary confirms, he mentions visiting Ropsha on 5th, 12th, 15th, 19th and 30th of July. ( on Aleksei's 11th birthday.)
I'd say that it was taken either on 19th or 5th, since only then NII mentions Sablin, Maria, Anastasia and Alexei, but it was most likely taken on 19th July 1915 (Sunday). He also mentions their delight when they saw the sea, walking along the sand, swimming and they obligatorily measured temperature of the water, which was 19 degrees of Celsius' scale on July 5th, when they first visited Ropsha.
In the photo, we see NII, MAA and Sablin.