وَالنَّهَارِ إِذَا جَلَّاهَا
And [by] the day when it displays it(the sun).
wan-nahaari idha jal-laaha.
jal-laaha – to give something thats already bright an exposure.
tajliyya – giving i.e. A lamp which is covered – giving it exposure so that its full brightness can be shown.
tajal-la – this was used in the classical Arabic, pre-Islamic era for a wife when she beautifies herself for her husband. Her full beauty is exposed.
And the Day when it – the day – gives her (the sun) the ability to show her full glory.
When we know that really – the sun is what causes the Day. Allah is illustrating that the sun is a beautiful creation which should be appreciated.