There are some good suggestions above. Below, are my recommendations for the first and last sentences. They might sound more 'native' this way.

11 of March was beginning of Pancake week. Maslenitsa, or "Pancake Week" began on the 11th of March.

In next Monday will begin Lent which proceed till Easter. Lent begins next Monday and continues until Easter. 

Pancake Week

11 Eleven of March [This is how we say but we write it as "March, 11" or "11 March"]was is the beginning of pancake blintz week. [Did you really mean this? http://en.wikipedia.org/wiki/Blintz]. Pancake week It is an old Slavic holiday with numerous customs. In During this time people bake make them pancakes [bake means to cook in the oven.  If it's on a stove etc, then it is "make"] and visit relatives. In this holiday This is the time that people are cheerfully see off the to say goodbbye to winter. Sunday - the last day of this "pancake week",  is also known as the Forgiveness Sunday. On this day, people go to visit relatives and friends. They ask for forgiveness from each other if any offence occured in the past. offended the word or an act. In next Monday will begin Lent will be in the following Monday and   goes on which proceed till Easter will be on Friday.

Pancake week

11 of March was the beginning of Pancake week. Pancake week is an old slavic holiday with numerous customs. In During this time people bake pancakes and visit relatives. In During this holiday people cheerfully see off the winter. Sunday, the last day of Pancake week is known as Forgiveness Sunday. In On this day people go to see relatives and friends. They ask forgiveness from each other if they have been offended the through words or an acts. In next Monday Lent will begin Lent which proceeds Easter.