You need to enter a leading zero for month and day to form a valid URI.
e.g. this does not work: https://wikimedia.org/api/rest_v1/metrics/pageviews/top/en.wikipedia/all-access/2015/10/1

The RESTBase documentation/description does not mention a leading zero is required.

1. go to https://wikimedia.org/api/rest_v1/?doc#!/Pageviews_data/get_metrics_pageviews_top_project_access_year_month_day
2. scroll down to the month or day parameters
3. notice how the description for each does not specify a leading zero is needed to make 2 digit days/months