Learn Russian: Согласно договорённости, мы встречаемся в среду. - According to the agreement, we meet on Wednesday.

The preposition согласно (according to) requires the Dative case. Since договорённость is a feminine noun ending in a soft sign, its Dative ending is -и. Note that while you might occasionally hear native speakers use согласно with the Genitive case, standard Russian grammar strictly dictates the Dative.

What is the difference between договорённость and договор?

A договор is usually a formal, written contract or legal treaty. A договорённость is a broader term for an agreement, arrangement, or mutual understanding, which is often informal or spoken. Here, the speakers likely just verbally arranged a time to meet.

Why does the sentence use the present tense встречаемся if Wednesday is in the future?

Just like in English (we are meeting on Wednesday), Russian can use the present tense of imperfective verbs like встречаться to talk about scheduled future events, arrangements, or fixed plans.

Is the comma after договорённости strictly necessary?

Yes, it is standard practice. In Russian, introductory prepositional phrases that establish the basis or reason for the main action (especially those starting with согласно, вопреки, or благодаря) are almost always set off by a comma when they begin a sentence.