1) when the market fluctuates over time or when the market goes down. So u can buy more units when price goes down and less when price goes up. Over the time u will hold more units and just wait until market goes up. This is called Dollar Cost Averaging. This often works based on these factors: (i) discipline in making regular contributions; (ii) time horizon; (iii) diversification across various asset classes; (iv) and professional management.
2) when the market has an upward trend with less fluctuations, lump sum investment is preferred.