It is best for training effect to cluster the long workouts together so I would suggest either Sat or Mon. It's part of periodizing the week and having better recovery overall.

If it's important to you to be fresh on the bike I would go for Monday. If timing is an issue and or fatigue you can do a split workout and get very good results. For eg, your long run is 8 miles, you could do 4 in the AM and 4 in the PM. I actually prefer a 5 and 3 split AM to PM.

If you are OK with a drop in performance on the bike. I would chose Saturday over Monday.

Hope that helps.