Paula's suggestion is nice and clear. However, by saying "..the video I uploaded today", the sentence isn't directed specifically at people who are watching on Monday. It is also directed at people that could be watching the video on Tuesday or Saturday, etc. This is because the meaning of the word "today" is relative to when you are watching. If I understand you correctly yujini, you want to say "Happy Monday" specifically to the people that are watching the video on MONDAY (the day that you upload it). So I would actually recommend using your original suggestion (with some small changes): "Happy Monday to those of you watching this video on the day that I upload it" I hope this helps!

Hi there! It is a bit confusing. How about writing the following: "Hi! Happy Monday to all of you watching the video I uploaded today".