I went to an open day for psychology on the 18th Feb, there were about 4 other dates but I sent the card with the dates on back to confirm which day I was going so I'm not sure when the others were, I know the one I went to was the only Saturday and the rest were wednesdays, I think the 18th of Feb was the second day avaliable so the other 2 were probably later but I don't know how late.
Sorry I can't be of more help, why not email the psych admissions and ask them incase its still in March? Its probably more likely, if there are any left to be in March as opposed to April. Or just ask them if you can visit anyway, go on a campus tour and see if you can have a snoop of the psych dept!
If you don't get a chance to visit the dept and want some opinions then I was really impressed with it myself (and I am the sort of person that has a critical eye and looks for flaws). The admissions tutors were really enthuasiastic and informative. The course sounded really interesting & impressed me the most out of all the places I visited, including Exeter and Royal Holloway. Its not essential as such to go to an open day, as all the information is in the booklet. Its just for them to show you what they can offer but personally I feel it was important for me to go to give me peace of mind so that I know what I'm choosing and can see it for myself but I guess its not the end of the world if you miss it because theres only so much you can find out from 11am - 3pm, it just gives you a feel for the place I guess. Just see if you can go there on a tour or something, I'm sure they'd try as hard as they can to accomodate you if are probably going to choose them.