You're looking too deep into the details.

As long as your product serves a need, does it in a unique way and does it well, then it doesn't matter what day you launch your SaaS project because it will grow organically.

If you REALLY want an answer then I would pick either a Tuesday or a Thursday. Mondays, Wednesdays, and Fridays are the days that news sites use to do "mass" updates on the latest and greatest.

Remember, it doesn't matter as long as your product has what I described above. Good luck! :)