The boss told me to mow the grass and I walked out for the day. Could I be fired Monday?

Depends on the terms of your employment. If you were employed to do a specific job, then you could object on those grounds, if however you were employed for general duties, then it would appear that you have some specific objection to cutting grass, or you have a chip on your shoulder about something.

If I was your boss under such later circumstances, you'd be out on your neck.

Well you will be fired...You should talk to you boss before you leave...

You could be is the short answer.

Yes. Seems like you may have a problem with following instructions. On second thought, you could tell him that there must be some misunderstanding because you went home and cut your grass...

Is that a normal job duty for you? You might be fired or he might forget about it, we don't know.

Not necessarily.

You might just get told to make more of an effort to follow directives in the future