documentclass[12pt]{article}

usepackage[ngerman]{babel}
usepackage[a4paper, text={16.5cm, 25.2cm}, centering]{geometry}
usepackage[sfdefault]{ClearSans}
usepackage[utf8]{inputenc}
setlength{parskip}{1.2ex}
setlength{parindent}{0em}


usepackage{tikz}
usetikzlibrary{shapes.symbols,shadows}

definecolor{bancolor}{RGB}{62,96,111}


 DeclareRobustCommanddatebanner[2]{begin{tikzpicture}
node[
  signal,
  signal from=north,
  signal pointer angle=150,
  fill=bancolor!75,
  drop shadow={shadow xshift=0pt, color=bancolor},
  rotate=-90,
  text width=2cm,
  text height=1.5cm,
  anchor=north west
  ] (banner) at (0,0) {};
% the text in the banner
node[
  anchor=south,
  text width=3cm,
  align=center,
  font=itshape]
  at ([yshift=1cm]banner.east) {#1 \ #2};
end{tikzpicture}}

begin{document}

tableofcontents

section{datebanner{2. April}{2018}}


vspace{64pt}

begin{tikzpicture}
node[
  signal,
  signal from=north,
  signal pointer angle=150,
  fill=bancolor!75,
  drop shadow={shadow xshift=0pt, color=bancolor},
  rotate=-90,
  text width=2cm,
  text height=1.5cm,
  anchor=north west
  ] (banner) at (0,0) {};
% the text in the banner
node[
  anchor=south,
  text width=3cm,
  align=center,
  font=itshape]
  at ([yshift=1cm]banner.east) {28. April \ 2018};
end{tikzpicture}

subsection{Vocabulaire}

subsection{film}


subsection{grammaire}

subsubsection{Si + ...}

subsection{orthographie}

end{document}

documentclass[12pt]{article}

usepackage[ngerman]{babel}
usepackage[a4paper, text={16.5cm, 25.2cm}, centering]{geometry}
%usepackage[sfdefault]{ClearSans}
usepackage[utf8]{inputenc}
setlength{parskip}{1.2ex}
setlength{parindent}{0em}

%added
usepackage{titlesec}
titleformat{section}{Largebfseries}{}{0pt}{}{}


usepackage{tikz}
usetikzlibrary{shapes.symbols,shadows}

definecolor{bancolor}{RGB}{62,96,111}

% changed, only one argument now
DeclareRobustCommanddatebanner[1]{begin{tikzpicture}
node[
  signal,
  signal to=south,
  signal pointer angle=150,
  fill=bancolor!75,
  drop shadow={shadow xshift=0pt, color=bancolor},
  minimum width=2.5cm,
  minimum height=3cm,
  align=center,
  font=normalsizeitshape
  ] (banner) at (0,0) {#1};
end{tikzpicture}}

begin{document}

tableofcontents

section[1. Mai 2018]{datebanner{1. Mai\2018}}

section[2. April 2018]{datebanner{2. April\2018}}

section[27. September 2018]{datebanner{27. September\2018}}


subsection{Vocabulaire}

subsection{film}


subsection{grammaire}

subsubsection{Si + ...}

%subsection{orthographie}

end{document}