The last day for official 23 man call up is on 4th June,2018 that is ten days before the kick off match.
Some countries already gave there final squads so will give the update to you guys in a few hours.
So these are the key dates of the tournament :
21st-27th May,2018   ⟹Rest period for the players 
28th May,2018           ⟹1st training camp for national squads
4th  June,2018            ⟹final date for 23 man call up 
14th June,2018           ⟹First Kick off of the FIFA World Cup