23 The phrase “the days of darkness” refers to the onset of old age (Eccl 12:1–5) and the inevitable experience of death (Eccl 11:7–8; 12:6–7). Elsewhere, “darkness” is a figure of speech (metonymy of association) for death (Job 10:21–22; 17:13; 18:18).
24 The term הֶבֶל (hevel) here means “obscure,” that is, unknown. This sense is derived from the literal concept of breath, vapor or wind that cannot be seen; thus, the idea of “obscure; dark; difficult to understand; enigmatic” (see I הֶבֶל; 210–11 I הֶבֶל). It is used in this sense in reference to enigmas in life (6:2; 8:10, 14) and the future which is obscure (11:8).

In other versions

ESVEnglish Standard Version

So if a person lives many years, let him rejoice in them all; but let him remember that the days of darkness will be many. All that comes is vanity.

NIVNew International Version (2011)

However many years anyone may live,

let them enjoy them all.

But let them remember the days of darkness,

for there will be many.

Everything to come is meaningless.

CSBChristian Standard Bible

Indeed, if someone lives many years,

let him rejoice in them all,

and let him remember the days of darkness, since they will be many.

All that comes is futile.

NLTNew Living Translation

When people live to be very old, let them rejoice in every day of life. But let them also remember there will be many dark days. Everything still to come is meaningless.

LEBThe Lexham English Bible

For if a man lives many years,

let him rejoice in all of them!

Let him remember that the days of the darkness will be many—

all that is coming is vanity!

KJV 1900King James Version

But if a man live many years, and rejoice in them all; yet let him remember the days of darkness; for they shall be many. All that cometh is vanity.

Public Domain