Here is how to correct any potential mistakes in the sentences:
Original : I am going to see an English film at the cinema this evening. Correction : There may not be any errors in this sentence; it is already correct.
Original : I look forward to seeing you at the weekend. Correction : There may not be any errors in this sentence; it is already correct.
Original : Both of the courses are on the same day, so I will have to miss one. Correction : There may not be any errors in this sentence; it is already correct.
Original : A TV company was at our school yesterday. Correction : There may not be any errors in this sentence; it is already correct.
Original : I'll come to your house during the holidays for a few days. Correction : There may not be any errors in this sentence; it is already correct.
Original : I want to know why the departure of the train was delayed until 8:15 pm. Correction : There may not be any errors in this sentence; it is already correct.
Original : I advise you to have a rest for a week and then start again. Correction : There may not be any errors in this sentence; it is already correct.
Original : I need to finish this letter because I must post it by 3 o'clock. Correction : There may not be any errors in this sentence; it is already correct.
Original : I enjoyed meeting your family during my short stay at your house. Correction : There may not be any errors in this sentence; it is already correct.
Original : I was travelling from Latvia on 10th December. Correction : The sentence is correct, but focusing on positive clarity, you could also rephrase it slightly to, 'I traveled from Latvia on December 10th.'
In conclusion, many of these sentences are already correct in standard English usage. It's important to understand the context and purpose to ensure clarity and grammatical correctness.