Then why don't non-Christians have their "bash" on, say 23rd, 27th., or any other day than 25th? It's all holiday time.
Because somebody, for whatever reason, decided upon December 25th as Christmas Day, which in itself is hopelessly wrong in any case.
You're forgetting that calendars are human creations - there are all sorts of different ones and are subject to alteration, tweaking and tampering (as happened in 1752 for example).