Can you make any sense of it? Here's the most pertinent information one more time, with mirror letters reversed:
a ms f Keita
on Dsembr 26 Keita is pr ....consut
Or, in non-text message style:
A message from Keita
On December 26th Keita is planning a concert
This was the first that I had heard of such an event, but he has been telling everyone he meets about it, with great seriousness. I asked him if he was going to sing but apparently no, it'll just be instruments. When I asked him what instruments he was going to play, he looked as me as if I were a fool and replied 'The ones we have'. So that'll be concerto for xylophone, tambourine and whistle then. That afternoon K started to spontaneously tidy up the living room, putting away all the toys and forbidding T to touch them. Why? In preparation for everyone coming over for the concert on the 26th of course. In the end I managed to persuade him that it might be a bit mean to not let T play with any toys at all until then, and that perhaps it'd be better to tidy up again nearer the time.
Today's advent calendar activity was 'Write a letter to Father Christmas'. I explained to K the idea of telling Father Christmas what you'd like as a present for Christmas, and he announced that he wanted... instruments, because he needs them for his concert and getting them for Christmas would be just in time for the 26th. I started to wonder if I could get a nice little toy piano or guitar but, in his actual letter to Father Christmas, we discover that he wants a horn... K also told H that he needs to translate his message into Japanese, so that he can invite people who don't speak English.
So, concert at our place on the 26th it is! Evbod cum!