Conventional is what you would expect based on what is common, at this time, in that culture.

Traditional has a sense of generations after generations.

For example, if you were getting married, a "traditional" wedding dress based on someone's culture might red, but in the U.S., a "conventional" dress is floor-length and white.

Also, families can have traditions. In a conventional Thankgiving dinner in the U.S., you have turkey, cranberry sause, stuffing, mashed potatoes, and sometimes either a green bean or yam cassarole. And pumpkin pie for dessert. But in YOUR family, the tradition might be to have ice cream sundaes afterwards, or instead of mashed potatoes, you have ... I don't know... potato pancakes as a side dish. The conventional dessert and side dish is not the same as your family traditions.

Grammar Geek wrote:

Conventional is what you would expect based on what is common, at this time, in that culture. Traditional has a sense of generations after generations. For example, if you were getting married, a "traditional" wedding dress based on someone's culture might red, but in the U.S., a "conventional" dress is floor-length and white. Also, families can have traditions. In a conventional Thankgiving dinner in the U.S., you have turkey, cranberry sause, stuffing, mashed potatoes, and sometimes either a green bean or yam cassarole. And pumpkin pie for dessert. But in YOUR family, the tradition might be to have ice cream sundaes afterwards, or instead of mashed potatoes, you have ... I don't know... potato pancakes as a side dish. The conventional dessert and side dish is not the same as your family traditions.

It's really a vivid explanation. Thank you Barbara.  ;-)