\documentclass[oneside]{book}%I guess
\usepackage{ifthen}\title{\Huge An unremarkable Diary\\\Large of unremarkable things\\ \small By an unremarkable person}% change this as you please
\date{}
\author{Mattia Mascarello}
\usepackage{titlesec}
\usepackage{blindtext}
\titleformat{\chapter}
{\Large\bfseries} % format
{}                % label
{0pt}             % sep
{\huge}   
% determine day ordinal from day number
\newcommand*{\dayOrdinalSuffix}[1]
{
	\ifthenelse{\equal{#1}{1}}{st}{
		\ifthenelse{\equal{#1}{2}}{nd}{
			\ifthenelse{\equal{#1}{3}}{rd}{
				\ifthenelse{\equal{#1}{21}}{st}{
					\ifthenelse{\equal{#1}{22}}{nd}{
						\ifthenelse{\equal{#1}{23}}{rd}{th}
					}
				}
			}	
		}
	}
}
\newcommand*{\justifyheading}{\raggedleft}
\renewcommand{\sectionmark}[1]{\markboth{#1}{}} % set the \leftmark
\titleformat{\section}{\normalfont\Large\bfseries}{}{0pt}{}
\titleformat{\subsection}{\normalfont\small\bfseries\justifyheading}{}{0pt}{}[{\titlerule[0.8pt]}]
\newcommand{\dayH}[3]{
	\subsection{#1 $#2^{\protect\dayOrdinalSuffix{#2}}$}
	\begin{flushright}#3\end{flushright}}
\newcommand{\yearH}[1]{\chapter{$#1$}}
\newcommand{\monthH}[1]{\section{#1}}
\begin{document}
	\maketitle
	
	\tableofcontents
\chapter{Preamble}
\blindtext
%set this as you please
\newpage
REPLACETHIS
\end{document}