John Gill Commentary Deuteronomy 27:2

"And it shall be on the day when ye shall pass over the Jordan unto the land which Jehovah thy God giveth thee, that thou shalt set thee up great stones, and plaster them with plaster:" — Deuteronomy 27:2 (ASV)

And it shall be, on the day when you shall pass over Jordan ,
&c.] Not the precise day exactly, but about that time, a little after they passed that river, as soon as they conveniently could; for it was not till after Ai was destroyed that the following order was put in execution; indeed as soon as they passed over Jordan, they were ordered to take twelve stones, and did; but then they were set up in a different place, and for a different purpose; see (Joshua 4:3Joshua 4:8Joshua 4:9) (8:31) ;

unto the land which the Lord your God gives you, that you shall set
up great stones ;
not in Jordan, as Jarchi, but on Mount Ebal, (Deuteronomy 27:4) ; nor had the stones set up in Jordan any such inscription as what is here ordered to be set on these:

and plaster them with plaster :
so that words might be written upon them, and be more conspicuous, and more easily read.