You probably think of tulips as Dutch. They reached western Europe from the Ottoman Empire in the sixteenth century, though, and in Turkey a stretch of the early eighteenth century is still called the Tulip Era. Every April Istanbul plants millions of bulbs in its parks for the Istanbul Tulip Festival, and you can see all of it without a ticket. It is a comfortable month for a first visit, too: the winter damp has lifted and afternoons reach about 17°C.
The festival lasts all month, but the flowers tend to peak around the middle of April, and a cold or warm spring can push that a week either way. With flexible dates, aim for mid-month and look at local reports in the days before you go. What should you pack? Something warm for evenings, which drop to around 9°C, and for the wind on a ferry deck, plus a small umbrella, since rain falls on roughly eight days.
Of the parks, Gülhane, beside Topkapı Palace, is the easiest to fit in. You will probably walk through it on the way to Hagia Sophia anyway, and its beds are enormous. For tulips that never droop, carry on down to Rüstem Paşa Mosque near the Spice Bazaar, a few minutes from the Eminönü piers, where the İznik tiles inside are painted with them. Mosques are open to visitors outside prayer times. Cover shoulders and knees and take your shoes off at the door. Women should bring a headscarf, though the big mosques usually lend them. Avoid Friday around midday, when the main weekly prayers are held.
Emirgan Park, up the European shore of the Bosphorus in Sarıyer, is the showpiece, its slopes planted between Ottoman-era pavilions. Weekends are busier, but that is when the free concerts are usually held. The ferry from Eminönü takes about an hour and is by far the nicest way there, but only a handful of sailings a day call at Emirgan, so check the Şehir Hatları timetable before you set out. Buses from Kabataş follow the coast road too, though traffic can slow them badly.
Buy an Istanbulkart when you arrive. It is the rechargeable card for ferries, trams, buses and the metro, and the machines at stations sell and top it up, which saves fumbling for change at every pier. On the boat, sit on the left going up. The ferry keeps to the European shore, so you get Dolmabahçe Palace and the waterside villages of Arnavutköy and Bebek all the way to the park.