There are many ways you can buy bitcoin, you can buy it from a friend or someone you know that has bitcoin and willing to sell at that time or you can buy it from a reliable exchange platforms. I don't know where you are from, you can get it from a local exchange in your country and also from international exchanges like: Binance and others. Is there any specific period of trading? The only period that is exempted is the hour of the day and not in the whole week or month. Trading can be done every week and every month. So feel free to trade every time.