A date is the one part of a plan you can set without doing anything. On Wednesday evening it gives you the feeling of a decision, and you pay nothing for it until Monday. You get the relief now and leave the effort for later.
Look at what your sentence leaves out. "On Monday I'll tackle the quotes backlog" names a day. It doesn't name the first file, the first line or the first ten minutes. On Monday morning you face the same open task plus a full inbox. So the date moves to the first of the month, then to January.
A date without a first move is just a postponement with a calendar entry.
Read the full article: https://impruvement.com/blog/i-ll-start-on-monday-why-a-fresh-start-date-keeps-moving