Student or Learner
If there is no accompanying reason, doesn't sentence (1) sound a bit short with the 'by this Friday' deadline time phrase? And does sentence (2) sounds better?
(1) "Let me know if you can come to my party by this Friday."
(2) "Let me know if you can come to my party as soon as possible / at your earliest."
Thank you for your time.
It's not 'by this Friday' which makes the sentence sound a bit short, but the lack of the word 'please'.
1. 'Please let me know by this Friday if you can come to my party.'
2. Don't omit 'by this Friday' if the deadline is important for any reason — such as catering or seating arrangements.