Usage should be self-explanatory, plug in the article, the date it appeared on DYK and the timeslot (this affects the calculation). There are also some caveats:
This tool uses data from the new WMF page views API.
The tool has a number of advantages over stats.grok.se; firstly it calculates figures for you, avoiding errors in reading the graph. Secondly it also includes mobile views (which adds about 10%!). However, data is only available from October 2015 onwards (so earlier DYK's will need to rely on the other stats tools).
D-1 = Previous Day
D0 = DYK day
D+1 = Following day
D+2 = 2 Days later
The calculation varies depending on timeslot (because the PM timeslot traditionally generates views on the D+1).
For DYK's in the AM (00:00 to 12:00) timeslot: D0 - ((D-1 + D+1)/2)
For DYK's in the PM (12:00 to 00:00) timeslot: D0 + D+1 - ((D-1 + D+2)/2)