Many Croatian time related expressions use the genitive case, and ovog tjedna 'this week', idućeg tjedna and ovog poslijepodneva are no exception.
See Croatian Days for the days of the week.
Croatian phrases ordered by topic.
A few first words. 1, A few first words. 2, Bathroom. Items found in the bathroom, Bedroom. Items found in the bedroom, Buying things. General phrases, Buyings things. Useful words, Countries, Communication problems, Conversation. Introductions, Conversation. Small talk. 1, Conversation. Small talk. 2, Conversation. Filler words, Conversation. Small talk. Sport, Conversation. Small talk. The weather, Days. General, Days of the week and seasons, Describing things. Colours, Describing things. Adjectives, Directions. 1, Directions. 2, Eating items, Eating phrases. 1, Eating phrases. 2, Emergencies, Family, Food and drink. General phrases, Food and drink. At the bar or café. 1, Food and drink. At the bar or café. 2, Food and drink. At the bar or café. 3, General phrases, Getting around. General phrases, Getting around. Train and bus, Getting around by taxi, Getting around by car, Health, Household items, Money, Months of the year, Numbers. 1 to 10, Numbers. 11 to 20, Numbers. 30 to 1000, Parts of the body, Places and buildings. 1, Places and buildings. 2, Question and size words, Sightseeing, Somewhere to stay. 1, Somewhere to stay. 2, Special occasions, Time, The house, Useful words, Useful words to recognize, Words to do with food. General, Words to do with food. Fruit, Words to do with food. Meat, Vegetables.