Because very few countries start their week on a Sunday, yet GitHub insists that is the only way to do it.

github-contributions-bloody-sunday.css

@namespace url(http://www.w3.org/1999/xhtml);
@namespace svg url(http://www.w3.org/2000/svg);
  
@-moz-document domain(github.com) {
  
  /* Only select sundays if: it is a full week, or the last week
     (I think this covers most current cases - first contrib of week may look messy) */
  svg|svg.js-calendar-graph-svg svg|g svg|g svg|rect:nth-last-child(7):first-child,
    svg|svg.js-calendar-graph-svg svg|g svg|g:last-of-type svg|rect:first-child { transform: translate( -13px, 78px ); }
  /* all other days must move as well */
  svg|svg.js-calendar-graph-svg svg|g > svg|g svg|rect { transform: translateY(-13px) }
  /* Move day labels up one row */
  svg|svg.js-calendar-graph-svg svg|text.wday { transform: translateY(-13px) }
}