can't we create a server task and link an on-demand scan to this server task?
Why would you link a task to a task?
or is it possible to create a client task and schedule it as run on particular day every week?
Yes, you can do this.
Can any one please explain
It's not very clear what you're trying to accomplish, or where you're having difficulty. You may need to read the product guide to learn how to do what you want, or at least, to get the correct terminology so you can explain what you're trying to do.