When first learning to speak any language, the young may struggle to manage certain fine detail. A prefix can create difficulty.  

As a child I learned the meaning of the word last early.  For me it was simply the occasion when something of significance took place.  Like, the last time I had a bath.  Perfectly reasonable. 

Just one little complication.  I learned the meaning, but I did not learn the other potential versions.  As a result,  yesterday became lasterday.  Quite logical actually.  It was the last day before this day.  Couldn’t fault that.

Many did.  The first time I used the expression in first grade at school the laughter and the teachers admonishment were very hurtful.  I even tried to argue with the greatly amused classroom.  What was wrong with what I said. 

After all the term LAST is quite easily understood.  While Yester doesn’t appear on it’s own in the dictionary. 

Yester is an Old English geostran, of Germanic origin; related to Dutch gisteren and German gestern.  It is also the parish of Yester in East Lothian, Scotland.

The Dictionary definition of LAST is:

Adjective:        “coming after all others in time or order”  

                         “most recent in time; latest: last year”

Verb:                “on the last occasion before the present; previously”    

Noun:               “the last person or thing; the one occurring, mentioned, or acting after all others.  Like my last drink.

I Rest my case for

LASTERDAY.