It is ok to buy on the day when the fund rises, but it is risky to buy when it rises. If the fund has fallen for a period of time before, the fund is in an undervalued area, and it just rebounded and rose that day, then you can choose to buy.

If the fund belongs to the overvalued area, and the fund has been rising for some time, the risk of buying it on the same day will be greater, so it is not recommended to buy it, because the fund is likely to be at a high level.