HONG KONG – China shares posted their biggest gain in more than two months on Monday, led by the financial sector as investors cheered the most sweeping economic and social reforms that Beijing has announced in nearly three decades. The CSI300 of the leading Shanghai and Shenzhen A-share listings closed up 3.3 percent at 2,428.9 points, while the Shanghai Composite Index jumped 2.9 percent. This was their biggest daily gain since September 9. Gains came in markedly stronger volumes, Shanghai recorded its heaviest trading interest in about a month. China unwrapped its boldest set of reforms in nearly three decades on Friday, relaxing its one-child policy and further freeing up markets in order to put the world’s second-largest economy on a more stable footing.