Many people feel that because Saturday and Sunday comprise the weekend, Sunday is the last day of the week and Monday is the first. Plus, the workweek begins on Monday, and the anticipation (anxiety, dread, whatever you feel) of going back to work often overshadows the religious significance and history reflected in calendars.

As one user writes on Reddit:

The Monday-first calendar is also more in-tune with our subjective experiences. Sunday doesn't feel like the first day of the week, it feels like the last day of the weekend. For most people, Saturday and Sunday are days for rest/housework, whereas Monday is a day for school or business. Sunday morning feels feels like a less-relaxed Saturday morning, whereas Monday morning feels dramatically different than Sunday. Since there's a sharp change in how we feel on Monday morning, it makes sense to start the week then.