I'm not sure I understand your question, but its worth mentioning that the landsat image period is variable, with a maximum period of 16 days (it generally depends on latitude).

really dear samuel? I think landsat image period fixed on 16 days, but main part of my question is calculate  daily vegetation water content, if we have 2 scene at Landsat, tnx 

The landsat satellites take photos in strips as they pass over the Earth's surface. These orbit is organised so they pass over the entire Earth's surface every 16 days, but due to the width of the strips, there is some overlap. The overlap increases with latitude, so for example, there is only ~50% overlap in Iran, there is 100% overlap in Norway - so Norway has 100% coverage every 8 days. Iran has 100% coverage every 16 days, and 50% coverage every 8 days. Depending on the scale of what you are doing, having the occasional 8 day gap may help you.

I do not work with vegetation water content so I cannot comment directly, although I cannot think of how else you would extrapolate the daily values between landsat dates except by some form of interpolation.

Perhaps to help others who may be able to answer your question you could explain why you think interpolation will not work?

Samuel tank you  for your guidance about landsat orbit, scale my work is small, in the farm scale i want to detection water stress in the field using landsat and i need 100 coverage, in other words i want to monitoring  water in the canopy of the plant, when we do irrigation, we can not do interpolation, because plant reached the saturation situation and changed the result between two image, in other words irrigation interrupt the result.