At 4. Ds's birthday is shortly before Christmas though, so an older 3 year old might have a good grasp on it. It was still exciting at 2 and 3 but that whole anticipation of the big night/day where he comes down the chimney and leaves gifts to find in the morning didn't really happen until 4. At 2 my DS looked at Santa as some sort of ultimate cross over character. He fully recognised Santa, there he was with the Paw Patrol, with Mickey Mouse, with Donald Duck, with Scooby Doo, with the Octonauts, etc. He knew Santa was a Christmas guy and tried to decorate our tree with his Santa teddy while singing the Paw Patrol version of falalalala before I'd started decorating. So he got the very basic gist of the season and it was fun but he couldn't really understand it.
At 3 he had a better idea but still didn't fully understand it. He'd chosen what to ask for in September and when his birthday came in November he definitely muddled the two. Christmas decorations were all ready all over the city and in the shops by his birthday so when he got lots of birthday gifts he assumed Santa would be coming that day to give him his Batcave. In December any time he saw Santa, he assumed he'd get his Batcave there and then. The first party we were at and Santa appeared and gave him a small toy car and after his initial delight wore off, he came up to me worried Santa had forgotten his Batcave. When he was doing his advent calendar, he was really, really focussed on getting to day 10, because to him 10 was the big number that he would surely get his Batcave on. At 4 days to Christmas I remember him sitting in his buggy in Smyths trying to negotiate Christmas morning down to 2 days, instead of 4. So he got it to a much bigger degree but he didn't get it. He still loved it though and obviously learned a lot about it that year because Easter, which we treat as a bit of a mini Christmas, was only 3 months later and he had a good enough grasp of it to talk me into making an Easter countdown calendar and was in close to full anticipation mode for the Easter bunny. (He also picked his gift for the following Christmas on January 3rd and stuck with it for the year.)
4 really was the year that he knew how it went. That his birthday and Christmas were two separate things. He knew that seeing Santa at a party or on a special visit, meant he was just going to have a chat with Santa and tell him what he wanted. He knew that the advent calendar was 24 days long and that 10 had no particular significance. He was in full countdown and anticipation mode because he understood what and when he was waiting for. He properly enjoyed Christmas movies and events.