Meaning, when the sky will split open for the coming of the Angels. This meaning is indicated in the verse:

 وَفُتِحَتِ ٱلسَّمَآءُ فَكَانَتۡ أَبۡوَٰبٗا

(And the heaven is opened and will become gateways) (An-Naba: 19)

Thus, the splitting of the heaven is meant for the coming down of angels for the Account. They will descend to the earth and surround it, which will turn into another earth. Allah (swt) says:

 فَيَوۡمَئِذٖ وَقَعَتِ ٱلۡوَاقِعَةُ (15) وَٱنشَقَّتِ ٱلسَّمَآءُ فَهِيَ يَوۡمَئِذٖ وَاهِيَةٞ (16) وَٱلۡمَلَكُ عَلَىٰٓ أَرۡجَآئِهَاۚ

(Then on that Day, the Occurrence [i.e., Resurrection] will occur (15) And the heaven will split [open], for that Day it is infirm (16) And the angels are at its edges.) (Al-Haqqa: 15-17)