That must be Anglo-Saxon way of arranging calendar. Slavic people prefer fixed dates, I see. Ours is better - it is harder to forget when a special day falls in the year.
I think the reason Mother's Day and Father's Day are on Sundays and not on some fixed date each year is so families can celebrate these occasions together. It's true that many people now work on Sundays, but when these holidays were established, very few people worked on Sunday and it truly was a day for family, etc. It simply made sense then to set Sundays as the days when the holidays would be observed. Even today, out of the entire week, Sundays are the days when the most people are not obliged to work.