If you subscribed in December, particularly in the second half of the month - then there is every chance that with the holidays etc you didn't get put on the mailing list in time for the Dec mailing. So your first issue is the one posted in Jan (which happens to be mailed out tomorrow).
So you'll most probably get your first issue this week. But don't fret, your 12 or 24 issue sub doesn't start until your first mailed issue, rather than the date you applied.