In addition to the approach that uses the adjustwidth environment to change the width of the text block locally, you can also reduce the amount of inter-column white-space to reduce the overall width of the table and hence make it fit into the normal text block.
Specifically, I recommend that you (i) omit all vertical rules (because they take up some space), (ii) use the booktabs package to get well-spaced horizontal rules, and (iii) reduce the amount of inter-column whitespace. The third task may be achieved most easily, in my opinion, by using a tabular* environment instead of a tabular environment, setting \tabcolsep to 0pt, and using an @{\extracolsep{\fill}} directive -- I know, it's not exactly intuitive! -- to make LaTeX figure out how much intercolumn whitespace needs to to be inserted.
As in one of the other answers, I've specified the showframe=true option of the geometry package merely to show the width of the text block.
Image may be NSFW.
Clik here to view.
By the way, if one needs to specify that a table should have 21 columns of type l, it is not necessary to type
\begin{tabular}{ l | l | l | l ... l }
Instead, one may specify \begin{tabular}{ l *{20}{|l} } to denote 20 instances of "|l". As explained above, I think the table looks just as good -- actually, better :-) -- without any vertical rules, hence my specification *{21}{l}. If you simply must have vertical rules after the first column and then after every fifth column, say, you could specify \begin{tabular}{ l *{4}{|lllll} }.
Finally, when placing a tabular (or tabular*) environment in a float environment, it's better to use the table environment rather than the figure environment (as you did in your MWE).
\documentclass[letterpaper]{article}
\usepackage{booktabs}
\usepackage[margin=1in,showframe=true]{geometry} % remove 'showframe' option in real doc.
\begin{document}
\begin{figure}
\setlength{\tabcolsep}{0pt} % let LaTeX figure out amount of intercolumn whitespace
\begin{tabular*}{\textwidth}{@{\extracolsep{\fill}}*{21}{l}} % no vert. bars
\toprule
& 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 & 9 & 10 & 11 & 12 & 13 & 14 & 15 & 16 & 17 & 18 & 19 & 20 \\
\midrule
Day 1 & 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\\
Day 2 & 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\\
Day 3 & 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\\
\bottomrule
\end{tabular*}
\caption{\label{tab:text}Text Experiment}
\end{figure}
\end{document}
