You choose the frequency at which you want to make periodic investments: monthly, bimonthly, quarterly, half-yearly or yearly. The start date you select determines the day of the month on which the order will be placed. This must always be a trading day, otherwise the order will be postponed to the next trading day.  The execution depends on the market conditions at that time. The first order can take place no sooner than the day following the day on which the periodic order was placed. 

You can change your periodic order at any time, free of charge, as long as the order has not been executed. Orders that are ready to be executed on that day can no longer be changed.