If one sees clients until 5:30 or even a little after on Thursdays, is is permitted when Thursday night is the start of Purim? (Sundown in the area is 5:30) or is work not allowed?
Thank you for your question.
Seeing clients at the start of Purim involves two issues. The first one is that the minhag is not to do work on Purim, as the Shulchan Aruch says that a person who works on Purim will not see blessing from that work. The custom of not working on Purim applies during the day of Purim, the minhag however is to be lenient regarding working on the eve of Purim until sunrise.
The other issue is that when the time to read the megillah comes, we have to stop whatever we are doing and go hear the megillah. In fact even those learning torah, the greatest mitzvah, have to stop in order to hear the megilah together with everyone. Therefore, if seeing make sure that you stop seeing clients with enough time to get to shul to hear the megillah with everyone.
Have a happy Purim
Biur Halacha 696 D:H Ayin quoting Pri Megadim, Ashrei Hoish O:CH 3- Chap. 48-3.