S. R. Faruqi:

sāñg = spectacle, game

The idiomatic expression rāt thoṛī aur svāñg ( yā sāñg ) bahut is used when there would be a lot of work to be done and little time to do it in. Here Mir has made 'night' into a metaphor for youth, and the theme is outwardly moralistic-- that in youth whatever good work you can do, do it. (With regard to religion, youthful piety is particularly pleasing.)

But taking the fullest possible advantage of the idiom, by means of the word sāñg Mir has also suggested that he refers to the games and sports, the amusements, the pastimes and revelries and display of youth, not youthful piety and virtuous behavior.

A svāñg is a kind of show or dramatic play; the term is also used for a merely amusing spectacle. Thus Insha has said in a 'satire' on Mus'hafi:

svāñg nayā lāyā hai āj yih charḳh-e kuhan
laṛte huʾe āʾe haiñ muṣḥafī aur muṣḥafan

[today this ancient sky has brought a new drama--
they have come quarreling, the Writer and the Writeress!]

For the performance of a disguise artist [bahurūp] too, they say sāñg bharnā or sāñg karnā . In this regard, ban jo kuchh ban sake has acquired an additional pleasure-- that is, 'whatever disguise-artistry you can perform, perform it'.

[See also {558,2}; {1853x,3}.]

FWP:

SETS == IDIOMS
MOTIFS
NAMES
TERMS == SATIRE

There are two ways of reading the first line. The first is as a verse about youthfulness: 'While you're young, that's the best time to experiment and try things out, so don't waste your youth in prudence'. The second is as a verse about time: 'Hurry to do as much as you can at the first opportunity, because time will run out before you can do everything you want to do'. Either way, it's a potent 'Make hay while the sun shines' effect.

Then the second line-- how irresistibly, cleverly multivalent! Using a proverbial saying makes a particularly un-pin-downable 'A,B' effect. The punchy, popular-cultural word sāñg is of course required by the proverb (and also by the rhyme); but doesn't it then spread a kind of complex luster back over the first line? Is what young people do a 'mockery', a 'play', a 'farce', a 'scene' (see the definition above)? Is the speaker urging young peopleto get their youthful follies out of their system as quickly and completely as possible? Or does he enjoin them to be their best selves, and do their best work, before time runs out? Or perhaps it's a version of 'Ars longa vita brevis'. As so often, it's left to us to decide for ourselves.