It sounds very odd with the article. Is there a reason you want the article? If you want to emphasize that it's the next one, then say "I am planning to go to the next seminar, in July."
2. Yes, you would use the article. Both take "an." An ee, an ecks is how it would sound. These get "a": B, C, D, G, J, K, P, Q, T, U, V, W, Y, Z These get "an": A, E, F, H, I, L, M, N, O, S, X