What is the origin and development of the calender of the Western Church?

Some are based either on older Jewish festivals, such as pentecost that are to be no longer kept by Christians.

As far as I can tell, Christmas and Easter are both festivals that mock Christ and celebrate the victory of the false church over Christ and celebrates the killing of his disciples. Although different, elements of both are found in the book of Esther. Esther is not scriptural, never being referred to by Jesus or the apostles, and never mentioning God. The book of Esther appears to be an allegory of the murder of Jesus (Hamon) and the replacement of God's church (Queen Vashti) with one (Esther) in league with the Jews (Mordecai) who refused to worship Jesus. The festival instituted in Esther is Purim, not mentioned in the New Testament. Purim celebrates the death of the enemies of the Jewish nation (Esther 8:11-13) as well as the murder of Christ (Esther 7:10). Historically Christians recognised that Jews were mocking Christ in the keeping of this festival.

Esther 8:17 And in every province, and in every city, whithersoever the king's commandment and his decree came, the Jews had joy and gladness, a feast and a good day. And many of the people of the land became Jews; for the fear of the Jews fell upon them.

This appears to be where "good" Friday comes from. Notice also, it is about "fear of the Jews", nothing to do with the fear of God.

Christmas does not celebrate the birth of Christ but his death, that is why it is called Christ-mass, it is when Roman Catholics celebrate mass (communion) at midnight, literally (according to them) drinking blood. It is an occult reversal. It seems Easter is a reversal no mass being celebrated on Good Friday when he supposed to have died (which would be the day of the Lord's supper), but on Sunday when he is supposed to have risen.

The timing of Christmas comes from the timing of Purim in Esther, on the 13th, 14th, 15th of Adar (the twelfth month).

Esther 9:17-19 On the thirteenth day of the month Adar; and on the fourteenth day of the same rested they, and made it a day of feasting and gladness. 18 But the Jews that were at Shushan assembled together on the thirteenth day thereof, and on the fourteenth thereof; and on the fifteenth day of the same they rested, and made it a day of feasting and gladness. 19 Therefore the Jews of the villages, that dwelt in the unwalled towns, made the fourteenth day of the month Adar a day of gladness and feasting, and a good day, and of sending portions one to another.

Adar is described as being the twelfth month.

Esther 9:1 Now in the twelfth month, that is, the month Adar, on the thirteenth day of the same, when the king's commandment and his decree drew near to be put in execution, in the day that the enemies of the Jews hoped to have power over them, (though it was turned to the contrary, that the Jews had rule over them that hated them;)

Although it is celebrated around the 25th of December, this is accounted for by the 10/11 day shift from the Julian to the Gregorian calendar.

25th December is also close to the winder solstice, celebrating the death and rebirth of the sun. This appears to be why the wafers in Roman catholic masses are round, being symbols of the sun.

Secret sun worship, and other abominations have gone on in the religious community for some time. See Ezekiel 8. These were the elders that outwardly worshipped the Lord, but in secret worshipped the sun. It is no different today.

When Jesus gave us the Lord's supper, it was a reminder that we should be both eating his body and drinking his blood. These are figurative things that point at what we should be doing as Christians. Many stumble at this, not discerning the Lord's body, and not being partakers the new covenant, which you would do well to understand from scripture if you think you are in Christ.

Edit: I am no longer sure of this position. There maybe elements of truth in it. However, the feasts of Esther never seem to be encouraged or approved by God, even if God allowed them.

According to a number of sources, liturgical calendars (which is what the 'church year' is more formally known) have been around in some form since the fourth century, and were fully developed (though not in their present form) in medieval times. The liturgical seasons, and the celebration of saints days, were part of the same process. The Lutheran church and other Protestant churches retained the basic form when their division from the Roman church took place, dropping a large number of saints days but retaining others.

The actual seasons have remained largely unchanged since then, with most of the modifications (in both Protestant and Catholic circles) being to the saint days celebrated.