documentclass[a4paper]{book}
usepackage[T1]{fontenc}
usepackage[slovak]{babel}
usepackage{textcase}

usepackage{titlesec}

usepackage{libertinus}

titleformat{chapter}[block]
  {titlerule[1pt]addvspace{0pt}Largescshapebfseriescentering}
  {thesectionenspace}
  {0pt}
  {}
  [vspace{2pt}{titlerule[1pt]}]
titlespacing{chapter}%
   {0mm}% How much of titlerule line to remove from left
   {8mm}% Before titlerule
   {0mm}% After titlerule

DeclareTextFontCommand{ns}{% normal shape
  fontsecondaryshape{ulc}selectfont
}

begin{document}

letcleardoublepagerelax % just to make both titles go in the same page

chapter*{Veľkonočná nedeľa\ ns{alebo}\ Nedeľa Pánovho zmŕtvychvstania}

chapter*{Sobota\ ns{po Piatej pôstnej nedeli}}%

end{document}