המספרה הזאת פתוחה גם ביום שישי, ולכן אחותי הולכת לאותה מספרה בכל חודש.

In Hebrew, demonstratives like הזה / הזאת / האלה usually come after the noun:

• המספרה הזאת = this hair salon
• הספר הזה = this book

They also must agree with the noun in gender and number. Since מספרה is feminine singular, the sentence uses הזאת.

Because מספרה is a feminine singular noun, and adjectives in Hebrew agree with the noun they describe.

So:

• מספרה פתוחה = feminine singular
• חנות פתוחה = feminine singular
• ספר פתוח = masculine singular

The basic masculine form is פתוח, and the feminine singular form is פתוחה.

ביום שישי literally means on Friday.

It is made of:

• ב־ = in / on
• יום = day
• שישי = Friday (literally sixth)

So ביום שישי is the normal way to say on Friday in Hebrew.

In expressions like ביום שישי, Hebrew normally does not put ה before the day name.

So you usually say:

• ביום ראשון
• ביום שני
• ביום שישי

not normally ביום השישי when you simply mean the weekday Friday.

If you say השישי, that often sounds more like the sixth in some specific sequence, not just the weekday name.

גם means also / too.

Here it modifies ביום שישי, so the idea is that the salon is open on Friday as well.

• פתוחה גם ביום שישי = open on Friday too

Hebrew often places גם right before the word or phrase it emphasizes.

ולכן means and therefore / so / therefore.

It is built from:

• ו־ = and
• לכן = therefore / that’s why

So ולכן connects the two parts of the sentence: the salon is open on Friday, therefore the sister goes there every month.

Because Hebrew often expresses possession by adding a suffix directly to the noun.

• אחות = sister
• אחותי = my sister

The ending ־י means my.

This is very common, especially with family words:

• אחי = my brother
• אמי = my mother
• בתי = my daughter

You can also say האחות שלי, but אחותי is shorter and very natural.

הולכת is the present-tense feminine singular form of the verb הלך / ללכת (to go / to walk).

Because the subject is אחותי (my sister), which is feminine singular, the verb must match it:

• הוא הולך = he goes
• היא הולכת = she goes

Hebrew present tense is also used for habitual actions, so הולכת can mean goes in the sense of goes regularly.

Because after הולכת (goes), Hebrew normally uses the preposition ל־ to mark the destination:

• ללכת ל... = to go to...

So:

• לאותה מספרה = to the same hair salon

Without ל־, it would not work as the destination of goes.

Yes—אותה can mean two different things in different contexts.

Here, אותה means same and agrees with מספרה:

• אותו ספר = the same book
• אותה מספרה = the same hair salon

Since מספרה is feminine singular, Hebrew uses אותה.

But אותה can also mean her as a direct object pronoun:

• אני רואה אותה = I see her

In your sentence, it clearly means same, because it comes directly before the noun מספרה.

Yes, both בכל חודש and כל חודש can mean every month.

• בכל חודש = literally in every month
• כל חודש = every month

In this sentence, בכל חודש sounds completely natural and emphasizes the repeated occurrence across each month.

Because כל (every / all) usually does not take a noun with ה־ when it means every in a general sense.

So Hebrew says:

• כל יום = every day
• בכל חודש = every month
• כל שנה = every year

If you add ה, the meaning often changes and becomes more specific, like all the... rather than every....

Yes, this is very normal Hebrew sentence structure.

The sentence first gives a fact:

• המספרה הזאת פתוחה גם ביום שישי

Then it gives the result:

• ולכן אחותי הולכת לאותה מספרה בכל חודש

Using ולכן to connect cause and result is very common and natural.