We have started living together. In the past I have seen people talk of the Sunday roast or Friday fish as if it is some great cultural tradition. We have started doing similar things. I think that it is other drivers. Tradition is just blindly following on from what those before you have done. But really pragmatism is a better driver.
Following a routine makes things simpler and easier. So, shopping is simpler if you buy a similar set of food each week.
On the week days we are time poor compared to the weekends. So simple easy to prepare dishes are better for the week days. Complex or long-time-to-make dishes fit into the time schedule better on the weekends.
Finally, some predictability is good. So we eat some dished pretty much each week. But also variety is good but in two ways. We need a variety of food stuffs, but the can be cycled through. We also need new foods and dishes. I guess after a while some may become staples. This will happen to prevent a fish from being eaten so much you become sick of it. And also, as the kids get older their likes will change.