In the following example LaTeX (pdflatex and lualatex) does start on the first page with the typesetting but on the second, also the page is not broken correctly after August.
I would have expected this to happen with float objects but not with ordinary tables.
\documentclass[ngerman,12pt]{article}
\usepackage[T1]{fontenc}
\usepackage{booktabs}
\usepackage{babel}
\usepackage[left=1cm,right=1cm, top=1cm,bottom=1cm]{geometry}
\pagestyle{empty}
\begin{document}
\begin{tabular}{rrrrrrr} \multicolumn{7}{c}{April 2020} \\ \toprule
Mo & Di & Mi & Do & Fr & Sa & So \\ \midrule
& &1 &2 &3 &4 &5\\
6 &7 &8 &9 &10 &11 &12\\
13 &14 &15 &16 &17 &18 &19\\
20 &21 &22 &23 &24 &25 &26\\
27 &28 &29 &30 & & & \\
\bottomrule \end{tabular} \vspace*{2.5em}
\begin{tabular}{rrrrrrr} \multicolumn{7}{c}{Mai 2020} \\ \toprule
Mo & Di & Mi & Do & Fr & Sa & So \\ \midrule
& & & &1 &2 &3\\
4 &5 &6 &7 &8 &9 &10\\
11 &12 &13 &14 &15 &16 &17\\
18 &19 &20 &21 &22 &23 &24\\
25 &26 &27 &28 &29 &30 &31\\
\bottomrule \end{tabular} \vspace*{2.5em}
\begin{tabular}{rrrrrrr} \multicolumn{7}{c}{Juni 2020} \\ \toprule
Mo & Di & Mi & Do & Fr & Sa & So \\ \midrule
1 &2 &3 &4 &5 &6 &7\\
8 &9 &10 &11 &12 &13 &14\\
15 &16 &17 &18 &19 &20 &21\\
22 &23 &24 &25 &26 &27 &28\\
29 &30 & & & & & \\
\bottomrule \end{tabular} \vspace*{2.5em}
\begin{tabular}{rrrrrrr} \multicolumn{7}{c}{Juli 2020} \\ \toprule
Mo & Di & Mi & Do & Fr & Sa & So \\ \midrule
& &1 &2 &3 &4 &5\\
6 &7 &8 &9 &10 &11 &12\\
13 &14 &15 &16 &17 &18 &19\\
20 &21 &22 &23 &24 &25 &26\\
27 &28 &29 &30 &31 & & \\
\bottomrule \end{tabular} \vspace*{2.5em}
\begin{tabular}{rrrrrrr} \multicolumn{7}{c}{August 2020} \\ \toprule
Mo & Di & Mi & Do & Fr & Sa & So \\ \midrule
& & & & &1 &2\\
3 &4 &5 &6 &7 &8 &9\\
10 &11 &12 &13 &14 &15 &16\\
17 &18 &19 &20 &21 &22 &23\\
24 &25 &26 &27 &28 &29 &30\\
31 & & & & & & \\
\bottomrule \end{tabular} \vspace*{2.5em}
\begin{tabular}{rrrrrrr} \multicolumn{7}{c}{September 2020} \\ \toprule
Mo & Di & Mi & Do & Fr & Sa & So \\ \midrule
&1 &2 &3 &4 &5 &6\\
7 &8 &9 &10 &11 &12 &13\\
14 &15 &16 &17 &18 &19 &20\\
21 &22 &23 &24 &25 &26 &27\\
28 &29 &30 & & & & \\
\bottomrule \end{tabular} \vspace*{2.5em}
\end{document}