An observation will be carried out when it can be scheduled conveniently and without leading to large overheads, e.g. in target acquisition. This time is not known before a detailed schedule has been generated for a particular revolution a few days in advance.
However, there is the possibility to request an observation to be performed on a particular date (i.e. revolution) during the ISO mission. An example for this type of observing request is to observe a Mira variable at a certain phase in the light curve. The proposer specifies in the scientific justification when the target is to be observed, by giving the date.
Periodic observations are a generalization to fixed time observations. In the scientific justification, the proposer can specify the date of the first observation, if e.g. the phase of the phenomenon to be observed is important. Additionally, the number of observations to be carried out and the number of days between successive observations has to be given. In addition, the small fraction of sky accessible from ISO at any given time can render periodic observations impossible.
Warning: Fixed time and periodic observations result in a very inefficient schedule and narrow down the flexibility of ISO scheduling (see Sect. 4.5). Therefore a very strong scientific justification is necessary to support the observations.