Hey there!  Here's your corrections.  :)  Remember - you don't need to capitalize nouns in English, unless they're proper nouns (places and people).

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Saturday

Today I traveled by train (even better - I took a train) to St. Gallen. This is a city in the north of Switzerland and it's 1 hour 20 minutes away. 
I worked as a promoter for a new opening (generally, we say "grand opening" in this case) of a telecommunication shop. 
We were lucky that the weather was good and the temperature was not so cold, about 12 Celsius.  Many clients came to visit the new shop and see the new design. 

The day was really nice and it was also nice (I suggest using a different word here since you already said "nice" earlier in the sentence - maybe "pleasant" or "refreshing"?) to work on the fresh air. 

See you. (This phrase isn't necessary, usually we only say this in correspondence between two people, not in journals.  :)

Saturday

Today I travelled by train to Gallen St. This City is located in north of Switzerland and its 1 hour 20 minutes away from where I live.
I worked as a promoter for a new opening from a Telecommunication shop.
We were lucky, the weather was good and the temperature not so cold (about 12 celcius). Many clients came to visit the new shop, which has a new design now. 

The Day was really nice and it was also very nice to work outdoors.

See you!

*Travel BY, never with!

*Every time you say a place is x minutes/kiometers away, you must specify where is this place away FROM.

*In English you don't "have lucky", you ARE lucky!

Keep up the good work! ;)