Holy Days of Obligation are EVERY BIT AS IMPORTANT as are Sundays – and they are ALWAYS important. It is not accurate to say that “one year they are a Holy Day and next year they may not be”. What may change is that the observation of Holy Day may be moved to the next or the closest Sunday. This is a practice, as I understand it, that occurs most in the United States (“only in America”) with its excessive secularism (the new “religion” of choice) and abandonment of God, including by many “Cathoics”. I tolerate the practice, but I don’t agree with it. This is one of the deviations introduced by the modernists with Vatican II.
Giving God the adoration and worship He is due should NEVER be because we are obliged (required) to do so, but because we WANT to do so because we LOVE HIM and, therefore, WANT TO PLEASE HIM. This should also be our rule with respect to obeying His commandments.