Temperatures
Peter's website has the most useful resource for checking temperatures across the camino. However, weather patterns change and the one year is never the same as the last.
http://peterrobins.co.uk/camino/weather.html
Remember that you will cross three fairly high ranges starting from St Jean Pied-de-Port at a mere 160m, to 1200m before descending to Roncesvalles. The first 50kms are a roller coaster of ups and downs mostly through woodland until you reach Pamplona.
The next 200 + kms are varied - across Navarra and La Rioja vineyards with several hill ranges to cross.
The 230 kms between Burgos and Astorga are on the meseta (about 800m) and can be hot and dusty in summer and bleak and freezing in winter.
Through the mountains of LeĂłn - approx. 145kms from Astorga to Sarria - there are two ranges to cross taking you to the highest point at 1500m. Then you have the O'Cebreiro range at 1400m. The last 115 kms from Sarria are surprisingly arduous with sharp up hills and steep descents.
There is always a danger of snows in April in any of the high places on the camino (we missed snow by one day in the middle of May in 2002) so perhaps a lightweight, fleece jacket would be a good idea. I bought a 165g fleece pullover and had an open-ended zip put into it so it could double as extra long sleeve shirt and jacket.
Dress in layers - preferably with wool close to your skin as it doesn't sweat and will insulate you against the cold. During the day you can moult as it warms up!
The spring wildflowers should be blooming by May. Once the snows stop and the sun is shining everything will be lush and green. The large black and white European stork will return to their enormous nests on top of church towers and steeples.
Have a marvelous time!