documentclass{article}
usepackage{lipsum}
usepackage{xfrac}
usepackage[most]{tcolorbox}

NewDocumentCommand{currentschoolyear}{+o}{%
  IfValueTF{#1}{%
    sfrac{#1}{thenumexpr#1+1}
  }{%
    ifnummonth<9%
    sfrac{thenumexpryear-1}{theyear}%
    else
    sfrac{theyear}{thenumexpryear+1}%
    fi
  }%
}

%month=7
%day=31

%%%%%%%%%%%%%%%%%%%%%%%%%%%
% boîte resumé

defcouleurresume{red}%

%% style du titre « résumé du cours »
%tcbset{titreresume/.style={
%    boxed title style={
%        colframe=#2!50,%
%        colback=#2!10,%
%        coltext=blue,%
%    leftrule=1.5mm,rightrule=1.5mm,toprule=1.5pt,bottom=0pt,boxsep=1pt}}
%}
%
%% style des soustitres des résumés
%tcbset{soustitre/.style={
%    subtitle style={%
%    colback=#2!7,%
%    colframe=#2!50,%
%    boxsep=1mm,
%    fontupper={sffamilybfserieslargeselectfontcolor{#2!63!black}}}}
%}

% boîte créant la boîte résumé
NewTColorBox[auto counter]{resume}{+o+O{red}+m}{%
    minipage boxed title*=-106mm,
    attach boxed title to top center={yshift=-3mm,xshift=-linewidth/5},
    enhanced,
    nobeforeafter,
    lower separated=false,
    IfValueTF={#1}{%
      before upper={textcolor{#2!63!black}{currentschoolyear[#1] hfill{#3}hfillthetcbcounter/ref{nombre-de-resume}}},
    }{%
      before upper={textcolor{#2!63!black}{currentschoolyear hfill{#2}hfillthetcbcounter/ref{nombre-de-resume}}},
    },
    colframe=#2!50,%
    colback=white,%
    coltitle=#2!63!black,%
    leftrule=3mm,rightrule=3mm,
    toprule=2pt,bottomrule=2pt,
    left=3pt,right=0pt,top=3mm,
    fonttitle=sffamilybfserieslarge,
%    titreresume,
    boxed title style={
        colframe=#2!50,%
        colback=#2!10,%
        coltext=blue,%
    leftrule=1.5mm,rightrule=1.5mm,toprule=1.5pt,bottom=0pt,boxsep=1pt},
%    soustitre,
    subtitle style={%
    colback=#2!7,%
    colframe=#2!50,%
    boxsep=1mm,
    fontupper={sffamilybfserieslargeselectfontcolor{#2!63!black}}},
    title={normalsize Résumé}
}

usepackage{atveryend}
makeatletter
AfterLastShipout{%
  immediatewrite@auxout
  {stringnewlabel{nombre-de-resume}{{thetcb@cnt@resume}{}}}%
}
makeatother
begin{document}

begin{resume}[2017]{chapitre 1}
tcbsubtitle{Définition}

lipsum[1]
end{resume}

newpage

begin{resume}[2018,blue]{chapitre 1}

tcbsubtitle{Propriétés}
lipsum[2]
end{resume}

%showtcbcounter
end{document}