What would be a formula for calculating the block height at a certain point in time, approximative How would I calculate the block height for a date like next Christmas 0h midnight?
If you just mean a general formula, rather than something written in Solidity...
Something along the lines of:
last_block_number + ((future_time - time_now) / block_time)
time_now are in seconds since the epoch, and
block_time is the average expected block time.
The block time is currently hovering between 14 and 14.5 seconds.
So, for example, using a current epoch time of
1481214124, the epoch time of
1482537600 for midnight Christmas Eve, and the last block of
2771338 + ((1482537600 - 1481214124) / 14) = 2865872