Well according to the Prima strategy guide, snails are:
In Jan, Feb, March, October, November, and December, not available at all.
In April, May, June, July, August, and September, they are available through out the entire day but are seen infrequently.
In the official Nintendo Power strategy guide, it says that they are available April-Sept, all day, and are found only on flowers.
Not very helpful now that I look at it. But Aya's suggestion was good. I think I caught my snail when it was raining.