I've been looking into this some more...
There is no Russian name day for Aleksander in the whole of January or February:
https://www.namedaycalendar.com/russia/january/30The first name day in February is on Semyon's supposed birthday, 2nd, and that is for 'Inna'.
Aleksander, of course, is an ancient Greek name, all similar names derive from that origin.
The official St Aleksander Day, with the Roman Catholic Church, is August 30th. But there are a number of different Saint Aleksanders across the world, even if some of them share one of two feast days with the Catholic one but were different men venerated as Saints across different centuries. So they must still be acknowledging Aleksander's name day
as August 30th, and then the individual's actual birthday is their own birth or the date they became a Saint if this was unknown.
In Russian Orthodoxy there appears to be 2 Saint Aleksanders. Aleksander Nevsky, the 'grand prince of Vladimir', has his principal feast day is 23 November, and 30th August. Alexander Svirsky, another Russian Saint, is on April 17th, and August 30th.
List of the numerous Saint Aleksanders:
https://en.wikipedia.org/wiki/Saint_AlexanderAs to who wrote it, who is the author of the Unknown Diary, from elimination I'd suggest it was
Yuri Doro, who is otherwise missing a diary. It obviously can't be be Lyuda, Tibo or Aleksander as they are mentioned. Would Semyon call his new BFF 'Nick Tibo' and 'K Tibo' (K-olya Tibo?), I'm not sure, and likewise good pal Yuri K would surely have one familiar name for him, as would Rustem who had hiked with Tibo before and was familiar enough to be tactile. The writer joins Tibo in sewing the tent after Lyuda refuses, and they reference rules and things which have been 'decided' so this is unlikely to be written by Igor who would set those rules, but by someone who respects them, and his close friend Doro fits the bill. It's been claimed the handwriting doesn't match Zina.
Doro's hiking experience appears to suggest he only hiked once with Tibo, in 1956, although he isn't named in the list I can recognise the top of is head in the middle at the back of this photo 5.
https://dyatlovpass.com/gallery-1956-Chusovaya?lid=1All this leaves me with is the idea Aleksander probably told a fib and made out it was his birthday the day after Doro had a party, and nobody else was familiar enough to know otherwise. He appears to have only hiked with one of the hikers previously, with Igor.