The first one is only talking about the coming Monday, the Monday which is coming up this week. But if you say "On Mondays" you mean that EVERY Monday you practice for two hours. So if you say "On Monday" you mean just this week, but if you say "On Mondays" you mean it happens on a regular occurrence, every Monday.

There are slight differences of meaning. The series "Grammar in Use" from Cambridge has good, brief explanations. 1a - On Mondays (all Mondays), we practice (habit) for two hours. 1b - On Monday_ (all Mondays), we practice (habit) for two hours. (non-standard usage but common) 2a - On Monday (the upcoming Monday), we are practicing (certain future) for two hours. 2b - On Monday (the upcoming Monday), we are going to practice (planned future) for two hours. 2c - On Monday (the upcoming Monday, we will practice (general statement about the future) for two hours. 3 - On Monday (the upcoming Monday, we will practice (decision at the time of speaking) for two hours. Examples: How long will we practice on Monday? As usual, we are /practicing/going to practice/ for two hours. (certain or planned) Should we practice one hour or two on Monday? On Monday, we will practice for two hours. (decision)