Last Trading Day

It might be "of the year" or "of the option expiry period" or some other construct where people care what that day is.

You are a pro trader, working for Goldman, making a cool mil a month in profit participation, albeit with extreme volatility, and it's October 27th. You're $287 million ahead in trading gains; you keep 10% of the trading gains you make in bonus money paid out over 3 years. If you do nothing from now until year-end, you lock in that $28.7 million bonus. So you care a lot about the last trading day of the year.

Same deal if you sold someone a call option to buy Disney at $120 and the stock is hovering at $118 right now. You have 7 days remaining until the last trading day when that option expires. So you pray for a choppy, bad down market, keeping Disney below that call, so that you can have taken in all the premium they paid to buy the call option, but you don't have to, um, actually do anything.

That last trading day...matters.