Opportunity is bald behind.

My mother wrote this in my autograph album describing it as a Chinese proverb and explaining that it was traditional for a man in China to wear a plait (or pigtail), which you might catch him by once he had begun to walk away.  A missed opportunity by contrast cannot be recalled.  A search engine found versions of this from several European cultures, including Bulgaria.

Opportunity knocks.

This expression became the title of a popular television show.

Make the most of…
…enjoy or take advantage of a situation, a talent, one’s appearance or time, perhaps.

Do it while the going’s good is advice taken from horse-racing. The going is the ground on which the horses gallop. 

At an opportune time
could be an alternative here.

A window of opportunity
sounds rather small.

Strike while the iron’s hot comes from the blacksmith’s forge and means choose the best moment.

A modern blacksmith (Photo credit Wikipedia, Author Matěj Baťha Creative Commons Attribution ShareAlike 3.0)

Chance one’s luck or chance one’s arm may mean take a risk.

Chance would be a fine thing
implies lack of opportunity.

He knows a good thing when he sees it could describe an opportunist.

Not a ghost of a chance
means something is impossible.

Hard luck
is a form of commiseration when someone is disappointed about the way things worked out.

I wish you luck is frequently said by someone who does not really believe your plans will succeed.

Bad luck and good luck are often spoken about.  Sometimes the former is used to mean the same as hard luck, but really it means something unluckyIf we believe in a sovereign God, luck should not figure in our thinking or speaking, because it depends on chance, implying that events are random.

If lots are cast we might draw the short straw.  That’s a mixed metaphor as lots were originally pebbles.  In the story of Jonah God showed the sailors that Jonah was to blame for the storm when they drew lots God’s care about the outcome of these methods of making decisions is apparent throughout the Old Testament.   In Proverbs Chapter 16 verse 33 (NIV)  The lot is cast into the lap, but its every decision is from the Lord.