Девятого мая мы собираемся встретиться в центральном парке.

When Russian gives a calendar date, it usually uses:

• the ordinal number in the genitive
• the month in the genitive

So:

• девятое = ninth as a normal adjective in nominative neuter
• девятого = of the ninth
• мая = of May

Together, девятого мая literally looks like of the ninth of May, but it means on the ninth of May / on May 9th.

This is the normal Russian pattern for dates:

• первого января = January 1st
• второго февраля = February 2nd
• девятого мая = May 9th

Май is the dictionary form, or nominative case, of May.

In dates, the month is normally put in the genitive case, so:

• май → мая

That is why Russian says:

• девятого мая not
• девятого май

The same happens with other months:

• январь → января
• март → марта
• июнь → июня

Yes, in a way. Russian often uses a date expression like девятого мая by itself to mean on May 9th.

So the sentence can simply begin with:

• Девятого мая... = On May 9th...

You may also sometimes see:

• 9-го мая with the number written numerically, but the grammar is the same.

By contrast, Russian does use в with days of the week:

• в понедельник = on Monday

But with dates, the usual pattern is just:

• пятого апреля
• девятого мая
• двадцать первого июня

Here собираемся means we are planning, we intend, or we are going to.

The verb is собираться, which can mean:

• to get together / gather
• to get ready
• to intend / plan to do something

In this sentence, it has the intention meaning:

• мы собираемся встретиться = we are planning to meet / we’re going to meet

So it does not primarily mean that people are physically gathering right now. It means there is a plan or intention.

Because собираться often works like to plan / to intend, and it is followed by an infinitive describing the action planned.

Pattern:

• собираться + infinitive

Examples:

• Я собираюсь поехать. = I’m planning to go.
• Мы собираемся встретиться. = We’re planning to meet.
• Они собираются начать. = They’re going to begin.

So in your sentence:

• собираемся = are planning
• встретиться = to meet

This is a very common learner question, because these verbs are related but not identical.

1. встретить

This usually means to meet someone or to greet/receive someone as a direct object.

Example:

• Я встретил друга. = I met my friend.

2. встретиться

This is reflexive and often means to meet each other / to meet up.

Example:

• Мы встретились в парке. = We met in the park.
• Мы собираемся встретиться. = We’re planning to meet up.

3. встречаться

This is the imperfective partner and can mean:

• to meet regularly
• to be seeing each other / dating
• to happen / occur in some contexts

Example:

• Мы часто встречаемся. = We meet often.
• Они встречаются. = They are dating.

So встретиться is used here because the sentence is about one planned meeting event.

Russian often uses the perfective infinitive after verbs of intention when talking about a single completed result.

Here the intended result is:

• the meeting will happen

So:

• собираемся встретиться = we plan to meet up successfully / once

If you used the imperfective встречаться, it would usually suggest:

• repeated meetings
• habitual meetings
• an ongoing relationship

Compare:

• Мы собираемся встретиться завтра. = We plan to meet tomorrow.
• Мы собираемся встречаться каждую неделю. = We plan to meet every week.

So the perfective fits this sentence because it refers to one specific future meeting.

Because в means in, and when it shows location, the noun usually takes the prepositional case.

So:

• парк → в парке = in the park

The adjective must match the noun in case, gender, and number:

• центральный парк = central park
• в центральном парке = in the central park

That is why the endings change:

• центральный → центральном
• парк → парке

Normally, no. With парк, Russian usually says:

• в парке = in the park

Using на парке would sound wrong in standard Russian.

This is something you often just have to learn with each place word:

• в парке = in the park
• в школе = at school / in school
• на стадионе = at the stadium
• на площади = in the square / at the square

So for park, the normal preposition is в.

Yes, it could be omitted.

Russian often drops subject pronouns when the verb ending already makes the subject clear.

So both are possible:

• Девятого мая мы собираемся встретиться в центральном парке.
• Девятого мая собираемся встретиться в центральном парке.

The form собираемся already tells you the subject is we.

Including мы can add:

• clarity
• emphasis
• contrast

For example, if you want to stress we as opposed to someone else, keeping мы makes sense.

Yes, it could be rearranged. Russian word order is more flexible than English word order.

The original order:

• Девятого мая мы собираемся встретиться в центральном парке.

This is natural and puts the time first: On May 9th...

Other possible orders include:

• Мы собираемся встретиться в центральном парке девятого мая.
• В центральном парке мы собираемся встретиться девятого мая.

These all mean roughly the same thing, but the focus changes:

• Девятого мая... emphasizes the date
• Мы... emphasizes who
• В центральном парке... emphasizes the place

So the original sentence is normal and natural, especially if the date is the first piece of information the speaker wants to highlight.

The main stresses are:

• девЯтого
• мАя
• мы собирАемся
• встрЕтиться
• в центрАльном пАрке

A rough pronunciation guide:

• девЯтого мАя
• мы собирАемся встрЕтиться в центрАльном пАрке

This can be helpful because Russian unstressed vowels often sound weaker than they look in writing. Knowing the stress helps both pronunciation and listening.