Day length at start of September is about 14 hours, reducing to 11.5 hours by the end of the month. This is Scotland, so the weather could be anything- wet and windy or sunny (less likely). I have often found weather is better in April, May and September than it is in July or August. I have lived in Scotland for over 20 years and been visiting for over 35.
PS day length is duration between sunrise and sunset at Aviemore. Shading from the mountains will reduce this.