Bob is right, but it isn't quite as bad as he suggests. If you simply work through one of the editions of Wellington's Dispatches you will get almost everything you need and identify the gaps (usually when he was travelling). I've just done a random check for June 1810 and there are letters dated:

Celorico from 1st, 2nd, 5th, 6th, 7th, 8th, 9th, 10th, 11th, 12th, 13th, 14th, 15th, 16th, 17th, 18th, 19th, 20th, 21st, 22nd and 24th

(so not on 3rd, 4th or 23rd)

And from Almeida on 25th, 26th, 27th, 28th, 29th and 30th.

I've not done this systematically for the years in the Peninsula myself or I would post the results, but he is seldom hard to locate.

Rory