How is it ok to go to a non catholic wedding. Where did the church teach this.

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Holy Saturday is a pretty big deal in the Catholic Church.
I have to wonder: Would a "practicing Jew" accept a wedding invitation when the wedding day falls on Rosh Hashana?
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In situations like this, I like to think about what the Apostles would have done.
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For example, can you think of any one of the holy Apostles who would have attended a wedding the day after Good Friday?
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Furthermore, if one of them had been invited to such a wedding, perhaps his absence would speak louder than anything he could have said.
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St. Francis of Assisi believed that wearing the habit of his Order was a silent sermon.
He said in regards to Franciscans wearing their cassock, "Preach always, and if necessary, use words."
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If I were to go to a wedding on Holy Saturday, I would sit in the back, and not stand up or clap or anything like that, and I would respectfully give my "regrets" in advance to attend the reception. A Catholic "celebrating" or making merry on that day would be scandalous. And if you can't make merry on a particular day, it's not right to attend a wedding party.
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If they really wanted YOUR attendance they would have avoided putting you in the position of having to make this choice, by choosing a different day, such as one during the Church's wedding ceremony times. There are certain times of the year when Catholics are forbidden from having weddings and this is one of them.
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Canon 1258 of the 1917 Code of Canon Law:

"It is unlawful for the faithful to assist in any active manner, or to take part, in the sacred services of non-Catholics.  Merely passive or material presence may be tolerated on account of a civil office, or for the purpose of showing respect to persons, to be approved in doubtful cases by the bishop for grave reasons, at funerals of non-Catholics, at their marriages, and similar solemnities, provided there is danger of neither perversion nor scandal."

As long as you are passively attending a non-Catholic wedding, and not actively participating, it was permitted under the traditional law.

.
Holy Saturday is a pretty big deal in the Catholic Church.
I have to wonder: Would a "practicing Jew" accept a wedding invitation when the wedding day falls on Rosh Hashana?
.
In situations like this, I like to think about what the Apostles would have done.
.
For example, can you think of any one of the holy Apostles who would have attended a wedding the day after Good Friday?
.
Furthermore, if one of them had been invited to such a wedding, perhaps his absence would speak louder than anything he could have said.
.
St. Francis of Assisi believed that wearing the habit of his Order was a silent sermon.
He said in regards to Franciscans wearing their cassock, "Preach always, and if necessary, use words."
.
If I were to go to a wedding on Holy Saturday, I would sit in the back, and not stand up or clap or anything like that, and I would respectfully give my "regrets" in advance to not attend the reception. A Catholic "celebrating" or making merry on that day would be scandalous. And if you can't make merry on a particular day, it's not right to attend a wedding party.
.
If they really wanted YOUR attendance they would have avoided putting you in the position of having to make this choice, by choosing a different day, such as one during the Church's wedding ceremony times. There are certain times of the year when Catholics are forbidden from having weddings and this is one of them.
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That canon and the one I cited deal with two entirely different situations.

The canon I cited deals with attending weddings and funerals of non-Catholics, and it is at least permitted in theory to attend such events as long as you don't actively participate (although exact circuмstances may vary, as you pointed out in your other post).

The canon you cited, however, deals with attending the wedding of a Catholic who is getting married outside of the Church.  That is a very different situation, and it would be forbidden to attend.