I always make Wellington for Christmas Eve, but always with a fillet of beef - if yours is a roasting joint it will be nicer just roasted, wasn't clear from your post which you have.
Wellington is only a little bit fiddly and looks very special as if you worked on it for hours 
and you can do some things in advance.
Express version:
In the morning or whenever it suits, turn the fillet of beef in sizzling butter in a pan for a few minutes until nice and brown, don't forget the two ends.
Pop, with the melted butter from the pan, into a hot oven for 10 mins.
Leave to cool. Season with salt and pepper. You can spread the top with liver pate or chopped fried mushrooms or your preferred filling if you like. Then wrap in bought puff pastry (sheets are easiest). Seal the join with beaten egg (save the rest) and turn over with the sealed bit underneath so you have a smooth top.
It can be left at this stage until you need it.
OK, dinner time! Wash the top with rest of the beaten egg and lay a few pastry shapes along the centre to decorate, tiny leaves look nice.
Put in a hot oven for 40 mins which should give a nice medium rare result. Let it rest a little while before carving into slices.
Hope it works out if you go for it!