"And the moon when it becomes full," meaning: when it becomes uniform and complete in its roundness. This is part of the oath that begins from His saying: "So I swear by the twilight," and continues up to this point; He has sworn by all of these.
﴿وَالْقَمَر إِذا اتسق﴾ إِذَا اسْتَوَى فَاسْتَدَارَ، وَهَذَا قَسَمٌ مِنْ قَوْلِهِ: ﴿فَلا أُقْسِمُ بِالشَّفَقِ﴾ إِلَى هَذَا الْمَوْضِعِ أَقْسَمَ بِهَذَا كُله
