Do you see that each day is split up vertically in three parts? Each part resembles a part of the day, which you can click to add your priority. On desktop you can add your priorities directly in the overview.
Also, possible bug report: i've closed the tab (ctrl-w), then reopened it (shift-ctrl-t). It just opened a completely blank page. Firefox 63, nothing in JS console, unfortunately.
I wanted to try it in Chrome, but now it just loads Heroku's "Application error" page in every browser. I hope i didn't break it for everyone with that reopened tab. I've developed some Meteor apps years ago and i recall they were a bit… fragile. :)