It is a Day of violent physical disturbance that rends the heaven - Surah Al-Muzzammil ayat 18 in Tafsir

﴿ٱلسَّمَآءُ مُنفَطِرُۢ بِهِۦۚ كَانَ وَعۡدُهُۥ مَفۡعُولًا ﴾
[المُزمل: 18]

It is a Day of violent physical disturbance that rends the heaven to pieces

﴿السماء منفطر به كان وعده مفعولا﴾ [المُزمل: 18]

Dr Kamal Omar
With (the approach of) that (Day) the heaven is one which is about to break-up. His Promise has become fulfilled

Dr Laleh Bakhtiar
The heaven will be that which is split apart from it. His promise had been one that is accomplished

Dr Munir Munshey
(On that day), the sky will burst asunder; His promise is (simply) bound to be fulfilled

Edward Henry Palmer
whereon the heaven cleaves - its promise shall be fulfilled

Farook Malik
the heaven shall split asunder, and His promise shall be fulfilled

George Sale
The heaven shall be rent in sunder thereby: The promise thereof shall surely be performed

Maududi
the Day whose severity shall cause the heaven to split asunder? Allah´s promise is ever bound to be fulfilled