If you see X, think it to be Sanskrit "kSh".
If you see Z, think it to be Sanskrit Palatal Sh, or even other Sibilants like Cereberal Sh.
Google my blog for this related postings on Sibilants.
Christmas is basically Pagan Festival and seems to be spelled as "krishna - maasi". Google on this aspect of Krishna-Christ link. The meaning of Sanskrit "kRiShN" and "kRiST" mean the same, where one is Present Participle, and the other os Past Participle. Before Chritistinity, Roman Kings have been pictured with Vaishwanite Signs and worshiping Black Baby Christ.
The English "spy" and Sansktrit "spaz" may be related.