The Poles were supposed to be dropped in on the 19th, the third day, near Elden and attack the southern approaches to the bridge. A lack of gliders and transports meant they could not be dropped on the first. Bad weather and a further lack of transport delayed this drop until the 20th when the plan changed suddenly.
...a little before 7 a.m. on the 20th, Major-General Stanislaw Sosabowski [commander Polish 1st Independent Parachute Brigade] learned that his drop zone had been changed. The Polish Brigade would now land in an area a few miles west of the original site, near the village of Driel...
There was still very little news from Arnhem but, as Stevens briefed him on the new plan to ferry his troops across the Rhine from Driel to Heveadorp, it was obvious to Sosabowski that Urquhart's [commander British 1st Airborne Division] situation had taken a turn for the worse...
A Bridge Too Far, Cornelius Ryan, p 328-329
At this point on the 20th the drop zones in Arnhem, open fields, were overrun or under German observation. The remnants of the 1st Airborne Division had fallen back to Oosterbeek where they intended to hold onto a bridgehead until XXX Corps arrived, already late. Only Lt-Col Frost's 2nd Battalion had made it to the bridge, and they would lose it in the night.
Rather than cross the bridge, the plan was now to reinforce 1st Airborne's position with the Poles by using a ferry across the Rhine. Driel was near Oosterbeek across the Rhine, and there was a nearby ferry at Heveadorp large enough to carry tanks. The Poles were to take the ferry, use it to cross the Rhine, reinforce 1st Airborne, and hold until XXX Corps could cross in force.
The Polish drop on the 20th was aborted due to bad weather, but they were finally off on the 21st.
On this Thursday [the 21st], the outlook was slightly brighter... The Nijmegen bridge was safe and open; the tanks of the Guards Armoured were on the way--and, if the weather held, 1,500 fresh paratroopers of General Sosabowski's First Polish Brigade would land by late afternoon. If the Poles could be ferried quickly across the Rhine between Driel and Heveadorp, the bleak picture could well change.
A Bridge Too Far, Cornelius Ryan, p 370
But they found the ferry missing, the approaches covered with German AAA and fighters, and the drop zones crawling with Germans. The planned crossing of the Rhine failed.
Ironically a morning patrol by 1st Airborne discovered the ferry was missing, but the Poles were already en route. The ferry was found later, its moorings had been cut by artillery, just a few hundred yards beyond the search area.
The Germans recognized the threat to their flank and counter-attacked the Poles sapping German strength away from Oosterbeek. Over the next couple days they got a few hundred men across, only enough to cover the 1st Airborne's withdrawal.