The sequence is obviously
=k_a*Xmas^(k_b*Xmas)+k_c
where
k_a 11.863
k_b 0.2855
k_c 62.904

Use Goal Seek to find the value for 30000
=11.31
Enter this value at cell 20 of Xmas
Enter 1 into cell 1 and select all 20 cells.
Edit Fill Series Column