Answer by cmhughes for making a table fit on a page by moving it left

You could use adjustwidth from the changepage package which allows you to widen or shorten the page width from the left or the right.

I loaded the geometry package with showframe=true just so that you an see where the page boundaries lie.

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\documentclass{article}
\usepackage[showframe=true]{geometry}
\usepackage{changepage}

\begin{document}

\begin{figure}
 \begin{adjustwidth}{-2cm}{}
\begin{tabular}{l|l|l|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|c|}
\multicolumn{21}{l}{a)text}\\ \hline
& 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 & 9 & 10& 11& 12& 13& 14& 15& 16& 17& 18& 19& 20\\ \hline
day1&9.2&8.6&8.1&6.3&7.7&7.3&8.0&8.4&5.9&6.8&7.3&7.6&9.0&7.4 &6.6&7.0&5.3&7.0&8.0&5.2\\
day2&6.7&6.9&0&7.0&0&8.1&9.0&6.1&0&6.9&0&7.3&7.8&5.3&0&8.9&0&8.4&8.4&7.5\\
day3&0&5.2&0&0&7.5&0&0&8.2&0&0&5.4&0&0&8.3&0&0&7.7&0&0&7.3
\end{tabular}
\caption{\label{fig:text}text experiment}
 \end{adjustwidth}
\end{figure}

\end{document}