People search for a Saturday dua, a Tuesday dua, and find long texts set on attractive images. The plain truth is this: no supplication is transmitted from the Prophet ﷺ for any particular day of the week — not Saturday, not Sunday, not Tuesday, not Wednesday.
What circulates under those headings has no chain of narration, and most of it is modern composition. Spreading it is not harmless: attributing to the Prophet ﷺ what he did not say is a grave matter — 'Whoever lies about me deliberately, let him take his seat in the Fire' (agreed upon). Someone who passes it on believing it to be sunnah has erred, however good the intention.
That does not make the days of the week identical. Some carry an established virtue — but it is the virtue of a deed, such as fasting or sending salawat, not of a fixed form of words. That distinction is the whole matter, and it is set out day by day below.