The holding period starts the day after acquisition and is tested on the trade date of the sale. Crossing it can be worth a large share of the gain, which is why the calendar sometimes dominates the trade decision near the boundary.
Several mechanics reset or suspend the clock: a wash-sale-rule adjustment carries the old holding period forward, a protective-put or collar can suspend it, and shares received in an all-stock-deal usually inherit the original date while an all-cash-deal ends it.
Example: a $30,000 gain taxed at 35% short term costs $10,500. Held four more weeks to qualify at 15%, the tax is $4,500. The $6,000 saving is 20% of the gain, and the risk is four weeks of price exposure.
Related: cost-basis, tax-lot, tax-loss-selling, wash-sale-rule, all-stock-deal