Depends where in Thailand you are now. Go to the nearest Thai embassy/consulate (in neighboring country). Vientiane, Phnom Penh, Penang, etc.

Visa first, condo second

Get tourist visa in advance, this gives a 60-day entry, then extend at immigration 30 days for 1900 baht. Note, if you arrive without a visa (visa exemption) you get 30 days but can only extend 30 days not 60.

As usual, make sure to check the dates when they stamp you in.

60

If it's a Multiple Entry Tourist Visa (METV) you get 60-day entry, then can either extend it another 30 days at immigration OR leave Thailand and re-enter (free, except in some cases visa for neighboring country). The visa is valid for six months, meaning you can go and re-enter during that period. Even the end of validity you could still get another 60-day re-entry, with another possible 30-day extension.

Borders generally stay open every day. Immigration and consulates (and other government offices) close for national holidays.

The extension will be 30 days from the expiry date

If it's just a 90-day report you can go anytime 2 weeks before or 1 week after. But try to avoid the week after Songkran when it will be extra busy. Mondays and Fridays are also best avoided.