The plan was for Allied ships to stand offshore in deep water and send landing craft into the lagoon. The landing craft would go as far in as possible and discharge troops. At low tide in the early morning, the bombardment would begin. This isn't the sort of thing you can call off and reschedule if things go wrong, as they did. Once an attack is under way, the enemy knows your intentions.
Any delay merely gives the enemy time to reinforce or escape.
Unfortunately, the rumors of almost-tideless periods at Tarawa were true. November 20 was near last-quarter moon, resulting in a neap tide. Military planners knew about the risks of neap tide but did not realize the moon was unusually far from earth as well, weakening its tidal effects even more. Also, the Earth was only seven weeks from perihelion, meaning solar tides were unusually strong as well. Landing craft hit bottom hundreds of meters offshore and the Marines had to wade ashore under heavy fire.