Is the “day” in Hebrews 10:25 Sunday or the day Jesus returns?
"And let us consider one another in order to stir up love and good works, not forsaking the assembling of ourselves together, as is the manner of some, but exhorting one another, and so much the more as you see the Day approaching. For if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries" (Hebrews 10:24-27).
The article "the" before "day" means that one particular day is in the mind of the author. The admonition is to not forsake the assembling of Christians, which would occur on the first day of the week (Acts 20:7; I Corinthians 11:17-18; 14:23,26; 16:1-2). The reason for not abandoning the assembling, which encourages those who attend, is because the day (Judgment Day) is approaching. We ought to be concerned because willful sins will leave us facing the wrath of God on that day. We need all the encouragement we can get to stay out of sin.