Cold day

Yesterday was so windy and cold all day long. I needed to wear more clothes to stay warm.
I went to a school gymnasium to observe my two sons’ basketball practice. All the children practised very hard and they were sweating a lot. But I was very cold and couldn’t stay far from a gas stove!

Practice: noun 

eg バスケットボールの練習 = basketball practice 

Practise: verb 

eg バスケットボールを練習します = to practise basketball 

Cold day

Yesterday was very windy and cold all day long. I needed to wear more clothes to get warm.
I went to a school gymnasium to watch my two sons’ basketball practice. All the children practiced very hard and they were in a deep sweat. But I was very cold and couldn’t stay far from a gas stove!

(This is pretty good!  :))

                                                              Cold Day

Yesterday was a very cold and windy day. I had to wear layered clothing to keep/stay warm.
I went to the school's gym to watch my two sons’ basketball practice. All the children practiced very hard and they were in a deep sweat, but I was very cold and couldn’t stay far from a gas stove!

Note 1: Your use of the word OBSERVE is correct but I think it would sound a bit more natural if you used the word WATCH.

Note 2: If it was you son's school (as in they attend that school), you should change the indefinite article a  to the definite article the. But lets say your sons where playing at another school then its okay to leave the indefinite article a .

Note 3:People usually use the word GYM which is short for GYMNASIUM, although they are both equally correct, using the word shorten version (GYM) will make your english sound more relaxed and natural.

Very interesting entry!!! Good luck in improving your English!

Cold Day

Yesterday was so windy and cold all day long. I needed to wear more clothes to keep warm.
I went to a school gymnasium to observe my two sons’ basketball practices. All the children practiced very hard and they were in a deep sweat. But I was very cold and couldn’t stay far from a gas stove!