The first pulsar was discovered on August 6, 1967 by Jocelyn Bell, but it took a while (and the discovery of a second pulsar) to figure out what was being observed. So I'm guessing this discovery wasn't announced right away. When was the discovery of a pulsar first made public?
As noted by @BigBrother, the original article in Nature (Nature volume 217, pages 709–713 (1968)) has a publication date of 24 February 1968.
Now, it turns out there is a nice oral history by Jocelyn Bell Burnell at the American Institute of Physics web site, where she says:
We wrote up the first pulsar for publication, sent it to Nature. I think those were the days when the editor refereed all the papers himself. I also remember Martin Ryle calling him up sort of days in advance saying, “We’ve got something very interesting we’re sending to you. Look out for it.” And when the paper had been accepted but a few days before Nature appeared, Tony gave a colloquium in the Cavendish in Cambridge, and we gave it a rather titillating title, you know, “A New Kind of Radio Source,” or something like that. And word began to get around that something very interesting was happening, and people came in from everywhere. And I can remember that quite clearly. I can remember Fred Hoyle sitting in the front row. And at the end of colloquium Fred saying in his Yorkshire accent that, “This is the first I’ve heard about this things, but I don’t think they’ve white dwarves. I think they’re supernova remnants.” In other words, he had taken in the fact that we found these very compact objects. For reasons that I don’t recall why, he almost immediately could say he didn’t think white dwarves were good candidates, but we were dealing with something to do with supernova — which of course is what it turned out to be.
Another Nature paper, with a proposed candidate for the pulsar (gravitational lensing of a neutron star binary, interesting conjecture but not right) was published shortly afterwards on 30 March 1968 (5 weeks later) and has as a reference:
Hewish, A., Bell, S. J., Pilkington, J. D. H., Scott, P. F., and Collins, R. A., Nature, 217, 709 (1968), and Hewish, A., Cavendish Colloquium, 20 February, 1968.
The authors of this paper are from Cambridge, and thus at least one of them was likely at the Cavendish Colloquium at Cambridge. So one can strongly infer that the Colloquium that Burnell mentions is the one referenced on the 20th of February, 4 days before the issue date of Nature, and that this was the first 'public' announcement of the results.