According to the book it's a unique East African Muslim (though again not specifically religious) circumcision rite for boys usually from 8 to 13 performed between July and late September every year that signals a social transition of maturity (in the eyes of the community)
hmm will probably research more and expand on this in the future (it'd be great if anyone could share especially East Africans); the internet seems to be scarce on materials about this; it's all books and there are few non-academic articles
Here're some pictures for convenience (I hope they are accurate and not about some other rites...)
I would also argue not to translate jando as "circumcision rites" however, but rather keep the word since it's culturally unique to East Africa and circumcision doesn't express the other purposes of such rights that often exist (a signal of maturity, change in regard by community, maybe even health??)
There is a book in Swahili: Jando na Unyago, by S. J. Mamuya, East African Publishing House, 1972, 242 pages. Its focus was on human sexuality in East Africa, covering physical, social and moral aspects, and the intended audience appeared to be young people, say 10 to 18. It was fairly advanced for its time, though still very conservative (or just plain wrong) on some issues. Absolutely no sympathy for the LGBT community. No discussion of AIDS ukimwi obviously since it was written in the pre-AIDS era. My main point, however, which I seem to have saved until last, is that there was no mention of any specific religion in the book, as far as I can recall.
That picture is almost certainly from some other event (unyago maybe) since everyone appears to be female. (Though I can't see the person holding the picture very well.) Short hair is common for both sexes in Tanzania and typically required by schools. It can be confusing initially for people coming from a culture where short hair is almost exclusively worn by males. And after boys are circumcised it can be even more confusing, since they sometimes wear khanga to allow for more air circulation and reduce friction.
I could be wrong, but my understanding is that while most tribes that practice jando are predominantly Muslim, it is practiced by some Christians as well.